Gray-cheeked Thrush minimus subspecies (Catharus minimus minimus): COSEWIC assessment and status report 2023

Official title: COSEWIC assessment and status report on the Gray-cheeked Thrush minimus subspecies (Catharus minimus minimus) in Canada

Threatened

2023

COSEWIC assessment summary

Assessment summary – December 2023

Common name: Gray-cheeked Thrush minimus subspecies

Scientific name: Catharus minimus minimus

Status: Threatened

Reason for designation: This songbird subspecies breeds only in dense montane forests of the Newfoundland archipelago and south coastal Labrador, with small numbers on coastal islands of Nova Scotia and on the French islands of Saint-Pierre-et-Miquelon. This bird likely winters in forests of northeastern Colombia and northwestern Venezuela. It differs genetically, in colour, and in song from the larger northern subspecies that is widespread across boreal Canada. Once abundant across the island of Newfoundland, it is now largely restricted to high-elevation habitats and some coastal islands. This is primarily due to nest depredation by a squirrel species introduced to Newfoundland in 1963. Overall numbers of the subspecies are conservatively estimated to have declined by 26.7-30.4% over the past 10 years, with declines likely to continue into the future. Other low-level threats include ecosystem changes related to introduced herbivores and control of insect outbreaks, energy development, mining, and effects of agriculture and logging on wintering habitat.

Occurrence: Nova Scotia, Québec, Newfoundland and Labrador

Status history: Designated Threatened in December 2023.

COSEWIC executive summary

Gray-cheeked Thrush minimus subspecies

Catharus minimus minimus

Wildlife species description and significance

The Gray-cheeked Thrush (Catharus minimus) is a medium-sized thrush found in boreal coniferous and mixed forests of northern Canada, Alaska and Siberia. The Newfoundland subspecies (C. m. minimus) of the Gray-cheeked Thrush, hereafter known as the Newfoundland Gray-cheeked Thrush, is intermediate in size and coloration between the continental subspecies—the Northern Gray-cheeked Thrush (C. m. aliciae)—and the closely related sister species, Bicknell’s Thrush (C. bicknelli).

The breeding range of the Newfoundland Gray-cheeked Thrush lies almost entirely in Canada, with 92% located on islands and 90% on the island of Newfoundland. The Gray-cheeked Thrush remains one of the least-studied songbirds in North America due to its furtive habits; affinity for dense, tangled habitats; and northern breeding distribution.

Aboriginal (Indigenous) knowledge

All species are significant and are interconnected and interrelated. There is no species-specific Aboriginal Traditional Knowledge in the report.

Distribution

The Newfoundland Gray-cheeked Thrush is a neotropical migrant that breeds throughout the Newfoundland archipelago (including the islands of Saint‑Pierre‑et-Miquelon, a territory of France). It also breeds in limited areas along the north shore of the Gulf of St. Lawrence (historical records) and the Strait of Belle Isle in southern Labrador, and likely on a few small islands on the Atlantic coast of Nova Scotia. Although this songbird was once common at all elevations on the Island of Newfoundland, it is now largely restricted to montane habitats, likely due to the impacts of the introduced Red Squirrel (Tamiasciurus hudsonicus). Its winter range is poorly understood, but includes northeastern Colombia and northwestern Venezuela.

Habitat

The subspecies’ breeding habitat consists of windswept and stunted coastal conifer forest, montane old-growth conifer forest, regenerating clear-cuts and conifer scrub. A common structural feature of these habitat types is dense low cover, particularly of Balsam Fir (Abies balsamea). The subspecies’ winter habitat is poorly documented but includes pre-montane tropical forest in northern South America.

Biology

Little is known about the mating system of the Newfoundland Gray-cheeked Thrush, but it may include groups of breeding birds, with several males and females possibly tending individual nests, as occurs in some closely related thrush species. Egg-laying begins in mid-June, but other aspects of the subspecies’ breeding biology are poorly documented. Indirect evidence suggests that the Red Squirrel is a significant nest predator of the Newfoundland Gray-cheeked Thrush, and the two species rarely co-occur.

Population sizes and trends

The global population of the Newfoundland Gray-cheeked Thrush is estimated at about 91,000 mature individuals, based on the average of recent estimates from the Boreal Avian Modelling Project and Partners in Flight, and detailed calculations completed for this assessment using local survey data.

The Newfoundland Gray-cheeked Thrush has experienced one of the most severe declines ever documented by the Breeding Bird Survey (BBS) in Canada. Breeding numbers in areas sampled by the BBS fell by -99.5% (95% CI: -99.9, -97.5) during the 1974 to 2019 period, and a cumulative decline of -71.4% (95% CI: -90.7, -21.4) was noted over 10 years (2009 to 2019). However, the BBS overestimates the actual decline, as the survey does not sample the high elevations and coastal islands where the population persists. More conservative estimates that correct for the areas not covered by the BBS suggest an overall long-term population decline of -93.5 to -95.5%, and a decline of -26.7 to -30.4% over the past 10 years.

Threats and limiting factors

The main threat faced by the Newfoundland Gray-cheeked Thrush is the presence of invasive, non-native species, particularly Red Squirrels, which were first introduced to Newfoundland in 1963 and later reached other islands in the thrush’s breeding range. Lower-level threats include energy development and mining; ecosystem changes related to introduced herbivores and the control of insect outbreaks; and the ongoing loss and degradation of wintering habitat due to the conversion of forests for agricultural use and deforestation from logging. The impacts of these threats are exacerbated by the fact that the subspecies’ breeding areas are largely confined to islands, as well as by variable productivity and recruitment due to low survivorship and moderate population fluctuations.

Protection, status and ranks

The Newfoundland Gray-cheeked Thrush is not listed under the federal Species at Risk Act (S.C. 2002, c. 29), although the subspecies and its nests are protected under the Migratory Birds Convention Act, 1994 (S.C. 1994, c. 22). In 2005, the Newfoundland Gray‑cheeked Thrush was listed as Vulnerable under the Newfoundland and Labrador Endangered Species Act (S.N.L. 2001, c. E-10.1), and subsequently uplisted to Threatened in 2014. It does not have any special status in Quebec, and its status has not been assessed in Nova Scotia. NatureServe ranks this subspecies as G5T4 globally (Apparently Secure subspecies); on the island of Newfoundland, where only the minimus subspecies is present, the species is ranked as S2B, SUM (Imperiled Breeder, Unranked Migrant).

Technical summary

Catharus minimus minimus

Gray-cheeked Thrush minimus subspecies

Grive à joues grises de la sous-espèce minimus

Viu; Ittipornipippiok (Labrador Inuktitut; from Austin 1932)

Range of occurrence in Canada: Nova Scotia, Quebec, Newfoundland and Labrador

Demographic Information

Generation time (usually average age of parents in the population)

2.31 years

From Bird et al. 2020

Is there an [observed, estimated, inferred, or projected] continuing decline in number of mature individuals?

Yes

Observed and inferred decline based on Breeding Bird Survey (BBS) data, corroborated by other sources

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 3 years [or 1 generation; whichever is longer up to a maximum of 100 years]

33.2% decline over 3 years according to BBS data; more conservative population-wide estimate of

9.1 to 10.6% decline over 3 years

Estimated decline from 2016 to 2019 based on BBS data; more conservative estimate accounts for areas where the population persists but that are not sampled in the BBS (see Fluctuations and trends).

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 5 years [or 2 generations; whichever is longer up to a maximum of 100 years]

55.4% decline over 5 years according to BBS data; more conservative population-wide estimate of

15.1 to 17.6% decline over 5 years

Estimated decline from 2014 to 2019 based on BBS data; more conservative estimate accounts for areas where the population persists but that are not sampled in the BBS (see Fluctuations and trends).

[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last 10 years [or 3 generations; whichever is longer]

71.4% decline over 10 years estimated by BBS: more conservative population-wide estimate of

26.7 to 30.4% decline over last 10 years

Estimated decline from 2009 to 2019 based on BBS data; more conservative estimate accounts for areas where the population persists but that are not sampled in the BBS ( see Fluctuations and trends).

[Projected, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the next [10 years, or 3 generations, up to a maximum of 100 years]

Projected continuing decline of 3 to 70% over 10 years

Inferred from anticipated effects of threats with a High - Medium impact

[Observed, estimated, inferred, projected, or suspected] percent [reduction or increase] in total number of mature individuals over any period of 10 years [or 3 generations; whichever is longer, up to a maximum of 100 years], including both the past and future (up to a maximum of 100 years in future)

Estimated and projected continuing decline of 3 to 70% over 10 years; up to 71.4% based on decline over last 10 years

Recent decline projected to continue into future at similar rates, as inferred from anticipated effects of threats with a High - Medium impact.

Are the causes of the decline clearly reversible?

No, key causes are not reversible in the short term.

Introduced species are now established on the Island of Newfoundland and are likely to persist, although eradication may be possible on some smaller islands; some human-caused impacts on habitat may be reversible.

Are the causes of the decline clearly understood?

Yes, major causes are understood.

Multiple lines of evidence indicate that the introduction of the Red Squirrel (and other species) is the primary cause; loss or degradation of winter habitat is also suspected.

Have the causes of the decline clearly ceased?

No

Introduced species established on many islands and may spread to others; habitat loss on wintering grounds is likely ongoing.

Are there extreme fluctuations in number of mature individuals?

No

Extent and occupancy information

Estimated extent of occurrence (EOO)

267,700 km²

Calculated based on minimum convex polygon drawn around known breeding season occurrences since 2009

Index of area of occupancy (IAO), reported as 2x2-km grid value.

3,965 km² (95% CI 3,105 to 5,185 km²)

Estimated using an occurrence model developed for this assessment; using a 2 km x 2 km grid may result in a higher estimate

Is the population “severely fragmented”, that is, is >50% of individuals or >50% of the total area “occupied” (as a proxy for number of individuals) in habitat patches that are both (a) smaller than required to support a viable subpopulation, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. No
2. No

The population is not severely fragmented, although the isolation of breeding groups has likely increased and the possibility of demographic rescue is reduced, compared to the 1970s.

Number of “locations” (use plausible range to reflect uncertainty if appropriate)

Unknown, but likely >>10

The introduced Red Squirrel poses the most serious single threat. Since the Newfoundland Gray-cheeked Thrush is distributed across many coastal islands and mainland sites that are differentially at risk to this threat, each is considered as a separate location.

Is there an [observed, inferred, or projected] continuing decline in extent of occurrence?

Yes, inferred decline of 14.8% in last 10‑year period from historical levels

Decline inferred by comparison of extent of occurrence (EOO) calculated using all historical observations with EOO calculated using records from 2009 to 2019 period

Is there an [observed, inferred, or projected] continuing decline in index of area of occupancy?

Yes, observed and inferred

Decline based on BBS data showing widespread decline in occurrence

Is there an [observed, inferred, or projected] continuing decline in number of subpopulations?

Not applicable

No subpopulations

Is there an [observed, inferred, or projected] continuing decline in number of “locations”?

Unknown, but likely

This thrush has likely disappeared from many islands where the Red Squirrel was introduced, and the future spread of squirrels is expected to cause further declines.

Is there an [observed, inferred, or projected] continuing decline in [area, extent and/or quality] of habitat?

Yes, inferred and projected continuing decline in quality of habitat

The breeding habitat is largely intact, but its quality has declined in most of the range due to colonization by the Red Squirrel and it is locally degraded by introduced herbivores. Land use has likely reduced the area and quality of winter habitat.

Are there extreme fluctuations in number of subpopulations?

Not applicable

No subpopulations

Are there extreme fluctuations in number of “locations”?

No

Are there extreme fluctuations in extent of occurrence?

No

Are there extreme fluctuations in index of area of occupancy?

No

Number of mature individuals (in each subpopulation)

Quantitative analysis

Is the probability of extinction in the wild at least 20% within 20 years [or 5 generations], or 10% within 100 years]?

Unknown

Analysis not conducted

Threats and limiting factors

Was a threats calculator completed for this species?

Yes, undertaken 27 February 2023 (See Appendix 1)

Overall assigned threat impact: High - Medium

Key threats were identified as:

IUCN 8 - Invasive and Other Problematic Species and Genes (Medium impact)

IUCN 2 – Agriculture and Aquaculture (Low impact)

IUCN 3 – Energy Production and Mining (Low impact)

IUCN 5 – Biological Resource Use (Low impact)

IUCN 7 – Natural System Modifications (Low impact)

IUCN 11 – Climate Change and Severe Weather (Unknown impact)

What limiting factors are relevant?

• The subspecies’ limited breeding distribution, largely confined to islands, makes it susceptible to impacts of introduced species
• Low survivorship and moderate population fluctuations may result in variable productivity and recruitment
• This neotropical migrant is exposed to weather-related threats on its long migration pathway

Rescue effect (natural immigration from outside Canada)

Status of outside population(s) most likely to provide immigrants to Canada.

Declining

The very small breeding population in the French territory of Saint-Pierre‑et‑Miquelon has declined concurrently with the Canadian population.

Is immigration known or possible?

Yes

Saint-Pierre-et-Miquelon is part of the Newfoundland archipelago, and immigration is likely possible.

Would immigrants be adapted to survive in Canada?

Yes

The small number of individuals that breed on Saint‑Pierre-et-Miquelon are presumed to be part of the same population that breeds on Newfoundland.

Is there sufficient habitat for immigrants in Canada?

Unknown

Although unoccupied habitat in Canada seems to be intact, it is likely degraded by the presence of introduced squirrels.

Are conditions deteriorating in Canada?

Likely

The continued range expansion of Red Squirrels on the Island of Newfoundland in response to habitat change and climate warming may result in further expansion into areas still used by thrushes.

Are conditions for the source (that is, outside) population deteriorating?

Likely

Numbers on Saint-Pierre-et-Miquelon are declining to close to extirpation levels (causes unknown).

Is the Canadian population considered to be a sink?

No

Canada supports > 99% of the global breeding population.

Is rescue from outside Canada likely, such that it could lead to a change in status?

No

The only potential source population is very small, and in steep decline.

Occurrence data sensitivity

Could release of certain occurrence data result in increased harm to the Wildlife Species or its habitat?

No

Status history

COSEWIC: Designated threatened in December 2023.

Status and reasons for resignation

Current status: Threatened

Alpha-numeric codes: A2bce+4bce

Reason for change of status: Not applicable

Reasons for designation: This songbird subspecies breeds only in dense montane forests of the Newfoundland archipelago and south coastal Labrador, with small numbers on coastal islands of Nova Scotia and on the French islands of Saint-Pierre-et-Miquelon. This bird likely winters in forests of northeastern Colombia and northwestern Venezuela. It differs genetically, in colour, and in song from the larger northern subspecies that is widespread across boreal Canada. Once abundant across the island of Newfoundland, it is now largely restricted to high-elevation habitats and some coastal islands. This is primarily due to nest depredation by a squirrel species introduced to Newfoundland in 1963. Overall numbers of the subspecies are conservatively estimated to have declined by 26.7-30.4% over the past 10 years, with declines likely to continue into the future. Other low-level threats include ecosystem changes related to introduced herbivores and control of insect outbreaks, energy development, mining, and effects of agriculture and logging on wintering habitat.

Applicability of criteria:

A: Decline in total number of mature individuals:

Meets threatened, a2bce+a4bce

Meets Threatened, A2bce+4bce. The reduction in the number of mature individuals is conservatively estimated at 26.7 to 30.4% over the last 10 years, with continuing declines inferred in the extent of occurrence, observed and inferred in the index of area of occupancy, and inferred in habitat quality, all of which are anticipated to continue into the future, with nest depredation by introduced Red Squirrels being a major cause of population decline.

B: Small range and decline or fluctuation

Not applicable

Not applicable. The estimated extent of occurrence and the inferred Index of Area of Occupancy exceed thresholds.

C: Small and declining number of mature individuals

Not applicable

Not applicable. The estimated number of mature individuals exceeds thresholds.

D: Very small or restricted population

Not applicable

Not applicable. The estimated number of mature individuals exceeds thresholds.

E: Quantitative analysis

Not applicable

Not applicable. Analysis not conducted.

COSEWIC history

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2023)

Wildlife species

A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)

A wildlife species that no longer exists.

Extirpated (XT)

A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)

A wildlife species facing imminent extirpation or extinction.

Threatened (T)

A wildlife species likely to become endangered if limiting factors are not reversed.

Special concern (SC)
(Note: Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.)

A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at risk (NAR)
(Note: Formerly described as “Not In Any Category”, or “No Designation Required.”)

A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data deficient (DD)
(Note: Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” [insufficient scientific information on which to base a designation] prior to 1994. Definition of the [DD] category revised in 2006.)

A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

The Canadian Wildlife Service, Environment and Climate Change Canada, provides full administrative and financial support to the COSEWIC Secretariat.

Wildlife species description and significance

Name and classification

Current classification:

Class: Aves

Order: Passeriformes

Family: Turdidae

Genus: Catharus

Species: minimus

Subspecies: minimus

Common names:

English: Gray-cheeked Thrush (minimus subspecies), Newfoundland Gray-cheeked Thrush

French: Grive à joues grises (sous-espèce minimus), Grive à joues grises de terre-neuve

Indigenous: Labrador Inuktitut: Viu, Ittipornipippiok (Austin 1932)

The Newfoundland Gray-cheeked Thrush is a subspecies of the Gray-cheeked Thrush (Catharus minimus), which forms part of a recently diverged clade within the Catharus thrushes that includes Bicknell’s Thrush (C. bicknelli) and the more distantly related Veery (C. fuscescens) (Voelker et al. 2013; Fitzgerald et al. 2019; Whitaker et al. 2020). Wallace (1939) noted that Gray-cheeked Thrushes from Newfoundland were intermediate in size and coloration between continental Gray-cheeked and Bicknell’s thrushes. Subsequent authors recognized it as the nominate subspecies C. m. minimus (“Newfoundland Gray-cheeked Thrush”) and the continental form as C. m. aliciae (“Northern Gray-cheeked Thrush,” Whitaker et al. 2020; see also Todd 1963; Godfrey 1986; Ouellet 1993). Recent genetic assessments are consistent with this division of the species into two subspecies and indicate that the Newfoundland subspecies may have originated during the mid- to late Pleistocene in a disjunct Atlantic Shelf glacial refugium (Fitzgerald et al. 2017, 2019; see also Topp et al. 2013).

Bicknell’s Thrush was considered to be a subspecies of the Gray-cheeked Thrush, C. m. bicknelli, until 1995 (Ouellet 1993; AOU 1995). However, recent studies show that C. bicknelli is a sister species that diverged during the mid- to late Pleistocene (Topp et al. 2013; Voelker et al. 2013; Fitzgerald et al. 2019). Many earlier accounts and studies of the “Gray-cheeked Thrush” were actually based on Bicknell’s Thrush (for example, Wallace 1939; Erskine 1992).

The North Shore of the Gulf of St. Lawrence appears to be a narrow zone of mixing between the Newfoundland and Northern Gray-cheeked Thrushes (Fitzgerald et al. 2017); it is not known if any unmixed groups occur in this continental portion of the Newfoundland subspecies’ range. An apparent hybrid Gray-cheeked x Bicknell’s Thrush was also captured in this portion of the breeding range in southern Labrador in 2016, and identified using molecular genetic analysis (Fitzgerald et al. 2017).

Description of wildlife species

The Newfoundland Gray-cheeked Thrush is a medium-sized songbird, 17.7 to 18.1 cm in length (Ouellet 1993), with a mean mass of 32.0 g in males (range 29.0 to 36.2 g; Fitzgerald et al. 2017). Males and females are similar in appearance, with olive-brown upper parts (head, back, and tail); pale throat edged with black lines; cream- or buffy-coloured breast with dark spots extending down into a white belly; and a cold grey cheek (Wallace 1939; Whitaker et al. 2020). Juvenile birds look similar to adults but have light tips on the coverts and mantle feathers and more heavily spotted flanks, giving them a more spotted appearance overall (Whitaker et al. 2020).

The Newfoundland Gray-cheeked Thrush, Northern Gray-cheeked Thrush, and Bicknell’s Thrush look very similar in the field, but are not known to overlap regularly within their ranges during the breeding season. In the hand, body measurements can generally be used to distinguish between the species, although there is considerable overlap (Frey et al. 2008). The Newfoundland Gray-cheeked Thrush averages slightly smaller than the Northern Gray-cheeked Thrush, although there is considerable overlap in all morphological measures (see Table 1 and Designatable units).

^a Sources: (a) Ouellet 1993; (b) Fitzgerald et al. 2017.

^b An asterisk (*) indicates a statistically significant difference between the Newfoundland and Northern Gray-cheeked Thrushes.

Designatable units

Recognized subspecies in Canada:

Two subspecies of the Gray-cheeked Thrush occur in Canada: C. m. aliciae (Northern Gray-cheeked Thrush) occurs from eastern Siberia through Alaska to Labrador, and C. m. minimus (Newfoundland Gray-cheeked Thrush) occurs in southeastern Labrador, on islands of the Newfoundland archipelago and on a few islands on the Atlantic coast of Nova Scotia (Figure 1; Whitaker et al. 2020; (see Global range). Only the status of the Newfoundland Gray-cheeked Thrush is assessed here.

Figure 1. Approximate North American breeding ranges of the Newfoundland Gray-cheeked Thrush (pink) and the Northern Gray-cheeked Thrush (green), along with the migration (yellow) and winter ranges (blue) of the species as a whole. The breeding and migration ranges and broad winter range (light blue) were adapted from NatureServe (Ridgely et al. 2003), and the core winter range (darker blue) from Birds of the World (2022). Red triangles indicate the locations of museum specimens obtained during the non-breeding season that have been identified as the Newfoundland Gray-cheeked Thrush (n = 38; Allen 1900; Wallace 1939). The blue dots connected by dashed lines show the southbound migration routes of four GPS-tagged Newfoundland Gray-cheeked Thrushes (Whitaker et al. 2018), including one that, although the tag failed in Cuba, presumably continued to South America.

Designatable units (DUs)

The COSEWIC guidelines for recognizing Designatable Units (DUs) revised in 2020 indicate that those Dus recognized on the basis of being separate subspecies should also meet criteria for discreteness and evolutionary significance (COSEWIC 2020). Discrete means that there is currently very little transmission of heritable (cultural or genetic) information from other such units, and evolutionarily significant means that the unit harbours heritable adaptive traits or an evolutionary history not found elsewhere in Canada. Current COSEWIC guidelines set out two criteria for discreteness and two for significance, and a DU must meet at least one criterion from each category.

The Newfoundland and Northern subspecies of Gray-cheeked Thrush are considered to be separate Dus. Only the status of the Newfoundland Gray-cheeked Thrush is assessed here, and the evidence for recognizing that subspecies as a separate DU follows.

Evidence for discreteness

D1: Evidence of heritable traits or markers that clearly distinguish the Newfoundland Gray-cheeked Thrush from the Northern Gray-cheeked Thrush, indicating limited transmission of this heritable information to other DUs (COSEWIC 2020).

• Relative to the Northern Gray-cheeked Thrush, the Newfoundland Gray-cheeked Thrush typically has browner plumage on its remiges, back, and flanks; a creamier wash to the throat and breast; and a mandible with a brighter yellow base (Wallace 1939; Peters and Burleigh 1951; Todd 1963; Ouellet 1993; Marshall 2001). Wing chord, tail length, and culmen length average shorter than those in Northern Gray-cheeked Thrushes from western Labrador and northern Quebec, although with considerable overlap, and the pale base of the mandible is more extensive (Table 1; Ouellet 1993; Fitzgerald et al. 2017; see also Marshall 2001; Frey et al. 2008)
• Molecular genetic analyses reveal low variation at two nuclear loci and shallow mitochondrial divergence between the Newfoundland and the Northern Gray-cheeked Thrushes; no mitochondrial ND2 haplotypes were shared between individuals from Newfoundland (n = 41) and those from western Labrador, northern Quebec, Alaska, and Siberia (n = 24) (Fitzgerald et al. 2017; see also Topp et al. 2013). Pairwise comparisons of genetic variation using C. m. aliciae) were highly differentiated from those on the Island of Newfoundland (φst range: 0.314 to 0.601, p < 0.05), whereas φst values were low to moderate within subspecies (Fitzgerald et al. 2017)
• Investigations by Fitzgerald et al. (2019) of the genetic structure and biogeographic history of the Gray-cheeked Thrush showed that individuals from Newfoundland and southern Labrador (that is, C. m. minimus) were similar to each other (range: F_ST = 0.029 to 0.036), while those from western Labrador and northern Quebec (that is, C. m. aliciae) were segregated from the Newfoundland/southern Labrador birds but still showed greater similarity to them (range: F_ST = 0.025 to 0.031) than to C. m. aliciae from Alaska and Siberia (F_ST = 0.073). Nuclear and mitochondrial results were consistent with a scenario in which the Newfoundland and Northern Gray-cheeked Thrushes diverged during the mid- to late-Pleistocene, followed by subsequent nuclear gene flow in a narrow contact zone between geographically proximate Newfoundland and Northern Gray-cheeked Thrushes. Based on the slight but clear genetic differentiation, the authors’ results support recognizing the two subspecies (Fitzgerald et al. 2019; Taylor pers. comm. 2023)
• Marshall (2001, p. 59) compared the subspecies’ songs and found “constant and predictable” differences in part III of their four-part songs, but his descriptions are difficult to interpret, and the methods were not clearly presented. Marshall (2001, xiv, p. 59) described Part III of the song of the Northern Gray-cheeked Thrush from Siberia, Alaska, and Manitoba as “high-pitched and inflected upwards,” and that of the Newfoundland Gray-cheeked Thrush (from Newfoundland and La Tabatière) as “varied but not low.
• There is some evidence from the tracking of three GPS-tagged individuals that the Newfoundland Gray-cheeked Thrush may have a restricted winter range near the Sierra Nevada de Santa Marta range in northeast Colombia and northwest Venezuela, which is north of the main Gray-cheeked Thrush wintering area (Whitaker et al. 2018; Figure 1). Seven museum specimens were also collected during winter in the Sierra Nevada de Santa Marta mountains in Colombia, in 1898 and 1899, and subsequently identified as the Newfoundland Gray-cheeked Thrush (Allen 1900; Wallace 1939). In contrast, the species as a whole winters across a broad range spanning much of northwestern South America (Ridgely et al. 2003); a more restricted band of habitat across the same region but excluding the Caribbean Sea coast (Whitaker et al. 2020; Figure 1) may constitute the core of the winter range. Recent analyses based on eBird data (Fink et al. 2022) suggest that the species’ core wintering areas extend across the mountainous regions of Colombia, Ecuador, and Peru, but rarely reach the Caribbean coast

D2: Natural geographic disjunction between Newfoundland and Northern Gray-cheeked Thrushes, such that transmission of information (for example, individuals, gametes) between these “range portions” has been severely limited for an extended time and is not likely in the foreseeable future (COSEWIC 2020).

• Todd (1963) suggested that there may be a range gap between Gray-cheeked Thrushes breeding in central Labrador and those breeding along the north shore of the Gulf of St. Lawrence. If true, this would be a gap of 100 to 150 km (Fitzgerald et al. 2017; Figure 2). Although the survey effort has been limited, no Gray‑cheeked Thrushes were detected in this presumed range gap either by Fitzgerald et al. (2017) or in the 2010 to 2014 Quebec breeding bird atlas project (Robert et al. 2019), nor are there any other known records there (for example, eBird 2022), indicating that at best the species is rare in this region
• The 15 to 60 km wide Strait of Belle Isle provides some geographic disjunction between the Newfoundland and Northern subspecies, aside from the roughly 300‑km-long zone of secondary contact along the north (mainland) shore of the Strait (Figure 2), which is occupied by minimus birds that have some genetic characteristics in common with aliciae (Fitzgerald et al. 2017). The Strait of Belle Isle has been found to limit gene flow for a broad range of other bird species breeding on Newfoundland (see S1 below for examples)
• The Newfoundland Gray-cheeked Thrush May have a distinct, restricted winter range north of the primary winter range of the Northern Gray-cheeked Thrush, which may further limit mixing (Whitaker et al. 2018; see D1, above)

On the basis of this information, there is genetic, morphological and behavioural evidence to indicate that criterion D1 applies, and biogeographic and genetic evidence that criterion D2 likely also applies. Genetic analyses suggest that the Newfoundland and Northern Gray-cheeked Thrushes diverged during the mid- to late Pleistocene, although the genetic distance is slight and some secondary mixing may have occurred in a narrow contact zone along the north shore of the Gulf of St. Lawrence (See S1 below).

Evidence for evolutionary significance

S1: Direct evidence or strong inference that the Newfoundland Gray-cheeked Thrush has been on an independent evolutionary trajectory for an evolutionarily significant period, usually intraspecific phylogenetic divergence indicating origins in separate Pleistocene refugia (COSEWIC 2020).

• The genetic assessment of the Newfoundland (n = 41) and Northern Gray-cheeked Thrushes (n = 36) using the entire mitochondrial ND2 gene (1,041 base pairs) suggests that the subspecies diverged in the mid- to late Pleistocene, which is consistent with the hypothesis that the Newfoundland Gray-cheeked Thrush was isolated in a disjunct Pleistocene boreal refugium (Fitzgerald et al. 2019; see also Topp et al. 2013; Fitzgerald et al. 2017). The median age of the coalescent node for 34 Gray-cheeked Thrush haplotypes was 242,500 years (95% CI: 37,000 to 720,000 ky)
• Species distribution models predict the existence of a disjunct refugium of suitable habitat on the exposed continental shelf off the east coast of Newfoundland during the last glacial maximum ∼21,000 ybp (Fitzgerald et al. 2019), in accordance with the timeline for a split between the subspecies during the mid- to late Pleistocene. This is corroborated by molecular genetics studies of several other Newfoundland bird subspecies,^{Footnote 1} as well as trees such as Black Spruce (Picea mariana; Jaramillo-Correa et al. 2004; see also Jackson et al. 1997), which also exhibit endemism that may reflect isolation in an Atlantic Shelf Pleistocene glacial refugium and low genetic exchange with continental populations (Pielou 1991)
• The results of nuclear and mitochondrial genetic studies are consistent with a scenario in which the Newfoundland and Northern subspecies diverged, but with some subsequent nuclear gene flow between geographically proximate individuals (Fitzgerald et al. 2017, 2019). Samples from the southeastern coast of Labrador (n = 12) included an equal mix of ND2 haplotypes from Newfoundland and western Labrador/northern Quebec, while φst values and morphological characters were intermediate, indicating that the north shore of the Gulf of St. Lawrence is a narrow contact zone where mixing between subspecies occurs (Fitzgerald et al. 2017). This area represents just 8% of the breeding range of the Newfoundland Gray-cheeked Thrush (Table 2)

^a Potential habitat taken from the 2015 Canada land cover atlas (Government of Canada 2019); land cover classes presumed to be potential thrush breeding habitat include temperate or sub-polar needleleaf forest, sub-polar taiga needleleaf forest, mixed forest, and temperate or sub-polar shrubland.

S2: Direct evidence or strong inference used to infer that the Newfoundland Gray-cheeked Thrush possesses adaptive, heritable traits that cannot be practically reconstituted if lost—for example, persistence of the discrete population in an ecological setting where a selective regime is likely to have given rise to DU-wide local adaptations that could not be reconstituted (COSEWIC 2020).

• Most Newfoundland Gray-cheeked Thrushes have bred on oceanic islands, including the Island of Newfoundland (Table 2), over an evolutionarily significant period; the only exception is 8% of the breeding range that represents a mixing zone along the north shore of the Gulf of St. Lawrence (Table 2). Oceanic islands are ecologically distinct in ways that can have selective, evolutionary consequences for wildlife. For example, insular representatives occupy less diverse ecosystems, and so may experience reduced competition or predation pressure, making them vulnerable to extirpation or niche restriction as a result of species introductions (for example, Banks and Dickman 2007; Scheele et al. 2017)
• There are differences between the breeding habitats used by the subspecies. Compared to the Northern Gray-cheeked Thrush, the Newfoundland Gray-cheeked Thrush makes more limited use of deciduous thickets, and is typically associated with windswept coastal conifer thickets, conifer scrub, regenerating clear-cuts dominated by young Balsam Fir, and montane old-growth fir forests (Lamberton 1976a; Vassallo and Rice 1981; Thompson et al. 1999; Marshall 2001; Whitaker et al. 2015; McDermott 2021)
• Measured morphological differences between the subspecies, for example, smaller size of Gray-cheeked Thrushes from Newfoundland, with Newfoundland males having shorter wings, tails, and exposed culmens than males from western Labrador and northern Quebec (Table 1; Fitzgerald et al. 2017; Whitaker et al. 2020). This may reflect heritable adaptive traits linked to the use of these different habitat types, which are unlikely to be practically reconstituted if lost
• There is some evidence, based on a very small sample, that the Newfoundland Gray-cheeked Thrush may have high migratory connectivity, with a distinct, restricted winter range north of the presumed “core” winter range of the Northern Gray-cheeked Thrush (Whitaker et al. 2018; (see D1 above)

Therefore, there is genetic and biogeographic evidence of an independent evolutionary trajectory in support of criterion S1, as well as evidence suggestive of adaptive, heritable traits that are unlikely to be practically reconstituted if lost, in support of criterion S2.

The overall weight of evidence thus supports the Newfoundland subspecies of the Gray-cheeked Thrush as a distinct DU, as it meets the criteria for both discreteness (D1 and possibly D2) and evolutionary significance (S1 and possibly S2).

Special significance

The Newfoundland Gray-cheeked Thrush is largely endemic to Canada, with almost the entire breeding range located in this country (Table 3). The exception is a very small breeding group on the French territory of Saint-Pierre-et-Miquelon, which may now be close to extirpation (Etcheberry pers. comm. 2022; Jackman pers. comm. 2022).

^a Regions classified by elevation above sea level (m).

^b Occurrence rates on Newfoundland are based on McDermott et al. (2021), and assume rates are constant at elevations < 100 m and > 600 m; the occurrence rate for mainland Canada was based on the encounter rate during BBS surveys in the region; the occurrence rate for coastal islands was taken as the encounter rate from 94 point counts conducted on 14 islands in Placentia Bay and Bay of Exploits in 2017, using the protocol outlined in McDermott et al. (2021).

^c Estimated area occupied was calculated as the product of the occurrence rate and the area represented by each elevation class. The 95% CI was not available for continental areas or coastal islands, so was assumed to be proportional to that for the Island of Newfoundland.

^d The estimated number of mature thrushes was calculated by dividing the estimated area occupied by 0.055 km² (5.5 ha, the approximate territory size for Bicknell’s Thrush [Collins 2007]) to estimate the number of territories, and then doubling this value to estimate the number of individuals (assuming an equal sex ratio).

Aboriginal (Indigenous) knowledge

Aboriginal Traditional Knowledge (ATK) is relationship-based. It involves information on ecological relationships between humans and their environment, including characteristics of species, habitats, and locations. Laws and protocols for human relationships with the environment are passed on through teachings and stories, and Indigenous languages, and can be based on long-term observations. Place names provide information about harvesting areas, ecological processes, spiritual significance or the products of harvest. ATK can identify life history characteristics of a species or distinct differences between similar species.

Cultural significance to Indigenous peoples

There is no species-specific ATK in the report. However, the Newfoundland Gray-cheeked Thrush is important to Indigenous Peoples, who recognize the interrelationships of all species within the ecosystem.

Distribution

Global range

The Gray-cheeked Thrush is a neotropical migrant that breeds across the northern North American boreal forest from Alaska to Newfoundland (Figure 1; Whitaker et al. 2020), as well as in eastern Siberia (Kretchmar 1997), and winters in northern South America. The breeding range of the Newfoundland Gray-cheeked Thrush is almost entirely restricted to eastern Canada (> 99%; Newfoundland and Labrador, Nova Scotia, Quebec), although a very small number still breed in the French territory of Saint-Pierre-et-Miquelon off the south coast of Newfoundland (Table 2; Figure 2). During the breeding season, individuals occur locally throughout the Newfoundland archipelago and on a few islands on the Atlantic coast of Nova Scotia (Figures 2 and 3). A narrow zone of mixing with the Northern Gray-cheeked Thrush occurs along the north shore of the Gulf of St. Lawrence and Strait of Belle Isle in extreme eastern Quebec and southern Labrador.

Figure 2. Breeding distribution of the Newfoundland Gray-cheeked Thrush in Canada (pink) and Saint-Pierre-et-Miquelon (blue). Yellow dots depict sites where the species was reported during the breeding period (June to August) from 1899 to 2008 (n = 899), and blue dots indicate observations from 2009 to 2021 (n = 466; (see Distribution: Extent of occurrence and area of occupancy for sources; note that many historical observations are obscured by recent observations). The current EOO is bounded by the dashed dark grey line, and the red polygons delineate the key geographic elements of the population’s breeding range in Canada: Newfoundland, the north shore of the Gulf of St. Lawrence, and Nova Scotia. The approximate (presumed) breeding range of the Northern Gray-cheeked Thrush in Labrador (green) is shown for reference.

Figure 3. Newfoundland Gray-cheeked Thrush breeding evidence from breeding bird atlas projects conducted within the subspecies’ breeding range, including the second Maritimes atlas (2006 to 2010; Whittam 2015), the second Quebec atlas (2010 to 2014; Robert et al. 2019), and the first three years of the Newfoundland atlas (2020 to 2022; Newfoundland Breeding Bird Atlas 2023). The grey squares depict the 10 km x 10 km squares where atlassing was carried out, but where the Newfoundland Gray-cheeked Thrush was not recorded, and are only shown in those areas of Nova Scotia, Quebec and Labrador where the Newfoundland Gray-cheeked Thrush potentially occurs. The light orange, dark orange, and red squares depict squares having possible, probable, and confirmed evidence of the subspecies’ breeding, respectively.

The Newfoundland Gray-cheeked Thrush migrates along the Atlantic Flyway and the East Coast of the United States (Figure 1). Some individuals may cross the Caribbean Sea directly to reach South America, while others cross the Gulf of Mexico and then take an overland route through Central America, entering South America through the Darién region of northern Colombia (Whitaker et al. 2018). The winter range of the Newfoundland Gray-cheeked Thrush is not well understood but, according to the information available, the subspecies’ core winter range may be in northeastern Colombia and northwestern Venezuela (see Designatable units: D1). Northbound Gray-cheeked Thrushes (presumably both subspecies) depart northern South America, fly directly across the Caribbean, and often overfly the North American coast to continue far inland to the northern United States or even southern Canada before stopping (Gómez et al. 2017). The Sierra Nevada de Santa Marta mountains and the Darién region of Colombia may serve as critical stopovers for both subspecies in fall, and the former area is also important during spring migration (Bayly et al. 2013; Gómez et al. 2014, 2019). Migrating Gray-cheeked Thrushes (subspecies unknown) have also been detected in Honduras, Belize, and southern Veracruz, Mexico from mid-April through mid-May (Whitaker et al. 2020).

Canadian range

Most (∼92%) of the breeding range of the Newfoundland Gray-cheeked Thrush is found in the Newfoundland archipelago, including many of the hundreds of nearshore islands along the Newfoundland coast (for example, Fogo Island [Peters and Burleigh 1951], Gull Island [Vassallo and Rice 1981], and Baccalieu Island [Wells and Montevecchi 1984]). Additionally, the subspecies appears to breed on a few islands along the mid-Atlantic coast of Nova Scotia, with singing males recorded on Raspberry, White Head, and Harbour Islands (Whittam 2015), representing < 0.1% of the breeding range. Birds on Harbour Island were confirmed to be members of the Newfoundland subspecies through genetic analysis (Fitzgerald et al. 2017), and this species was observed there again in 2016 (eBird 2022). Surveys have been limited, so the subspecies may occur on more of the roughly 170 small islands along this coast. There is no evidence of breeding on mainland Nova Scotia or Cape Breton, despite considerable survey effort (Whittam 2015). The Lower North Shore of the Gulf of St. Lawrence, including the Strait of Belle Isle, may represent a more significant breeding area (up to ∼ 8% of the breeding range), with historical records along the coast from La Tabatière, Quebec, east to Cape Charles, Labrador (Townsend and Allen 1907; Austin 1932; Todd 1963; Marshall 2001). However, this area is a zone of introgression with the Northern Gray-cheeked Thrush (Fitzgerald et al. 2017), and it is unknown if any unmixed Newfoundland Gray-cheeked Thrushes occur in this continental portion of the range. The historical and contemporary distributional limits on Quebec’s Lower North Shore and in Labrador are poorly described, and the inland limit was arbitrarily delineated for this assessment based on Todd (1963). However, since all records occur along the coast (Figure 2), this mainland portion of the range may be restricted to a narrower coastal zone.

Several sources indicate that the distribution of the Newfoundland Gray-cheeked Thrush has been greatly reduced and become more localized across much of its breeding range. Historical reports and BBS data indicate that the subspecies was common or abundant throughout much of coastal Newfoundland until the early 1980s, including southwestern Newfoundland, the Great Northern Peninsula, the northeast coast, the Avalon Peninsula, and various sites along the south coast (Figure 2; Peters and Burleigh 1951; Lamberton 1976a, b; Vassallo and Rice 1981; Marshall 2001). It was abundant in high-elevation habitats (Mactavish 1988), and Lamberton (1976a) identified it as the fifth most abundant breeding bird species in the highlands of Gros Morne National Park. Historical BBS data indicate that the subspecies had a bimodal elevational distribution, with lower abundance at intermediate elevations (Figure 4; Robineau-Charette et al. 2023). In stark contrast, recent BBS surveys, eBird reports, initial data from the Newfoundland Breeding Bird Atlas, and systematic surveys confirm that the Newfoundland Gray-cheeked Thrush is now absent or rare below elevations of around 400 m throughout most of the Island of Newfoundland (Figures 2 to 4; Fitzgerald et al. 2017; eBird 2022; McDermott et al. 2023; Newfoundland Breeding Bird Atlas 2023). Evidence indicates that this pattern is primarily a consequence of nest predation by introduced Red Squirrels (Tamiasciurus hudsonicus), which are now abundant at lower elevations (see Current and Future Threats: Invasive and Other Problematic Species and Genes).

Figure 4. Comparison of detection rates for the Newfoundland Gray-cheeked Thrush on silent point counts, as a function of elevation on the Island of Newfoundland. Historical (4,200 observations; orange line) and recent (7,150 observations; green line) detection rates during three-minute point counts on 17 BBS routes (Robineau-Charette et al. 2023), and during four-minute point counts by McDermott (2021; blue line) on the eastern slope of the Long Range Mountains in 2016 and 2017, are shown. Splined curves were fitted using generalized additive models with a binomial detection function and a smoothing factor of k = 4. The large confidence interval at low elevations for the McDermott (2021) data resulted from a single thrush detection at 105 m above sea level (ASL), whereas no others were observed below 350 m ASL.

The Newfoundland Gray-cheeked Thrush is still relatively abundant at elevations greater than around 400 m above sea level (ASL) on Newfoundland, including the highlands of Gros Morne National Park and the surrounding Long Range Mountains (Hogan 1997; Whitaker et al. 2015; McDermott et al. 2023; D. Whitaker unpubl. data), although its incidence above 400 m also appears to be lower than in the past (Figure 4; Robineau-Charette et al. 2023). eBird and the Newfoundland Breeding Bird Atlas also confirm the subspecies’ continued presence in some localized coastal areas of Newfoundland (Figures 2 and 3), including the northern tip of the Great Northern Peninsula, the eastern shore of Placentia Bay, the eastern shore of the Avalon Peninsula, and the Connaigre Peninsula (see also Fitzgerald et al. 2017). The Newfoundland Gray-cheeked Thrush still occurs on some coastal islands around Newfoundland, including Great (Big) Island in the Ramea group, the Merasheen islands in Placentia Bay, and Gull Island and Great Island in Witless Bay (eBird 2022; D. Whitaker unpubl. data).

Little information is available on potential changes in the distribution of the Newfoundland Gray-cheeked Thrush in Labrador, Quebec, or Nova Scotia. Townsend and Allen (1907) observed Gray-cheeked Thrushes from Forteau to Cape Charles in southern Labrador in July 1906, and a handful of observations were reported in the vicinity of La Tabatière, Quebec, and east towards the Labrador border from the 1930s through the 1950s (Figure 2; Gillet 1935; Gabrielson 1952; Todd 1963; Ouellet 1993; Marshall 2001). Marshall (2001) felt these individuals represented a populated locality extending from La Tabatière to Cape Charles. Recent observations by Fitzgerald et al. (2017), as well as BBS data (Table 4) and eBird checklists, indicate that the Gray-cheeked Thrush still occurs in moderate numbers in coastal Labrador south of Red Bay. However, there are no recent observations further northeast towards Cape Charles, and none along Quebec’s Lower North Shore since 1981 (see eBird 2022). Furthermore, despite over 700 hours of survey effort, this species was not found along the Lower North Shore during the second Quebec breeding bird atlas (2010 to 2014), suggesting that at best it is rare and localized in this region (Robert et al. 2019). No historical data are available for Nova Scotia, where the Newfoundland Gray-cheeked Thrush has only recently been discovered.

Population structure

The Newfoundland Gray-cheeked Thrush population does not exhibit any known genetic structure (Whitaker et al. 2020), although the area along the north shore of the Gulf of St. Lawrence and Strait of Belle Isle, from La Tabatière on eastern Quebec’s Lower North Shore to Cape Charles in Labrador, is a zone of intergradation with the Northern Gray-cheeked Thrush (Fitzgerald et al. 2017).

Extent of occurrence and area of occupancy

Current extent of occurrence

The current extent of occurrence (EOO) of the Newfoundland Gray-cheeked Thrush is approximately 267,700 km² in Canada, calculated from breeding season observations from 2009 to 2021 in the subspecies’ breeding range in Canada (n = 466; Figure 2).

Current index of area of occupancy

A minimum estimate of the current index of area of occupancy (IAO) for the Newfoundland Gray-cheeked Thrush in Canada of 620 km² was obtained by overlaying a 2 km x 2 km grid over all breeding season observations reported from 2009 to 2021 (n = 466; Figure 2). However, this approach likely underestimates IAO, as many potential breeding areas that are difficult to access have not been surveyed. An alternative, and more realistic, approach to estimating IAO uses estimated occurrence rates (the proportion of the landscape where the thrush is present during the breeding season) derived from quantitative survey data. Newfoundland comprises 90% of the population’s breeding range (Table 2), and elevation is the most important predictor of the subspecies’ current occurrence on the island (Whitaker et al. 2015). Survey results (McDermott 2021; 1960 points spanning 75 to 608 m ASL in western Newfoundland; Figure 4) were used to calculate the estimated area of occurrence for the Island of Newfoundland. This was based on occurrence rates for each 50-m elevation class from 100 m to 600 m (McDermott et al. 2021), assuming constant occurrence at elevations below 100 m and above 600 m. The estimated area occupied was then calculated as the product of the occurrence rate and the land area of each elevation class (Table 3).

A similar approach was taken for the rest of the breeding range of the Newfoundland Gray-cheeked Thrush, but (1) for nearshore islands (2% of breeding range), the occurrence rate was based on the encounter rate in 94-point counts conducted using the same survey method as McDermott (2021) on 14 islands in Placentia Bay and the Bay of Exploits in 2017 (D. Whitaker, unpubl. data); and (2) for the continental portion of the range on Quebec’s Lower North Shore and in Labrador (8% of breeding range), the occurrence rate was based on the encounter rate in nine Breeding Bird Surveys (BBS) conducted along three routes in the region from 2011 to 2019. Confidence limits were not available for these two areas, and therefore were assumed to be proportional to those for the Island of Newfoundland. The sum of these estimates yielded an estimated IAO in Canada of 3,965 km² (95% CI: 3,105 to 5,185 km²; Table 3); and the estimate of IAO obtained by overlaying a 2 km x 2 km grid over occupied sites would be even higher. However, the approach taken here likely slightly overestimates the occupied area, because occurrence rates for Newfoundland came from a study area that was specifically selected because it was known to have a robust thrush population.

Fluctuations and trends in distribution

The current estimate of EOO is 14.8% lower than the estimate that included historical observations within the subspecies’ breeding range (1899 to 2008; 318,877 km²; n = 899). These minimum values were based on a minimum convex polygon around current and historical observations reported in published accounts and publicly available databases (Townsend and Allen 1907; Gillet 1935; Peters and Burleigh 1951; Gabrielson 1952; Todd 1963; Lamberton 1976a, b; Ouellet 1993; Marshall 2001; SSAC 2010; Whitaker et al. 2015; Whittam 2015; Fitzgerald et al. 2017; Smith et al. 2020; McDermott 2021; eBird 2022). It is assumed that all currently occupied sites were also occupied in the past (for example, islands in Nova Scotia). Most of the inferred decline reflects the lack of contemporary observations in the portions of the subspecies’ range in Quebec and southeastern Labrador, despite recent search efforts there (Fitzgerald et al. 2017; Robert et al. 2019).

Robineau-Charette et al. (2023) assessed changes in Newfoundland Gray-cheeked Thrush occurrence along 17 BBS routes on the Island of Newfoundland, each of which had been surveyed at least three times between 1974 and 1984 and at least three times between 2001 and 2015, and where the species had been observed at least once. The Gray-cheeked Thrush was detected at 339 stops across all 17 routes from 1974 to 1984 (39.9% of stops; total 84 surveys), but at just 24 stops on eight routes from 2001 to 2015 (2.8% of stops; total 143 surveys). These routes all sampled areas below around 400 m in elevation, suggesting a 93% reduction in occurrence since the mid-1980s at these elevations. As 81.8% of the population’s breeding range on the Island of Newfoundland is found at elevations below 400 m, this suggests that 76% of the total area occupied by the subspecies since the 1970s has been lost. Given that the BBS suggests that the Newfoundland subspecies’ population is still declining (see Population Sizes and Trends), it is likely that that IAO is also still declining.

Biology and habitat use

Information on the biology and habitat use of the Newfoundland Gray-cheeked Thrush is taken primarily from the Birds of the World account on the species (Whitaker et al. 2020). Unless otherwise noted, the information refers to the species as a whole, rather than to the Newfoundland subspecies.

Although there are significant gaps in our understanding of the breeding biology and ecology of the Gray-cheeked Thrush, several recent studies have focused on the Newfoundland subspecies (for example, Whitaker et al. 2015, 2018; Fitzgerald et al. 2017, 2019; McDermott 2021; Robineau-Charette et al. 2023). Important advances have also been made in understanding the migration and stopover behaviour of the species as a whole (Bayly et al. 2013, 2016; Gómez et al. 2013, 2014, 2015, 2017, 2018, 2019).

Life cycle and reproduction

The longevity record for the Newfoundland Gray-cheeked Thrush is 6 years, 11 months (D. Whitaker and I. Warkentin, unpubl. data), equalling the longevity record for the Northern Gray-cheeked Thrush (Bird Banding Laboratory 2019). It is presumed that individuals first breed at one year of age and breed annually, like other Catharus thrushes. Generation time is estimated to be 2.31 years, based on calculations using annual adult survival, age at first reproduction, and maximum longevity (Bird et al. 2020).

Female Gray-cheeked Thrushes build cup nests on the ground or low in shrubs (Ouellet 1996). The only Newfoundland Gray-cheeked Thrush nests found were less than 2 m high along the trunk of a large Balsam Fir (n = 2) or in fallen trees (n = 2; Bent 1949; Birds Canada 2022). Nests of the Northern subspecies have been found in willow and alder thickets, at the base of small Black Spruce, and atop stumps and fallen tree trunks. The nest is composed of thin twigs, horsetails, rootlets, and strong grasses, with an inner lining of fine grasses or lichens. Although this has not been directly studied, the Gray-cheeked Thrush may nest in a semi-colonial fashion and have a polygynandrous breeding system, in which multiple males and/or females may attend a single nest, as has been reported in Bicknell’s Thrush (Townsend et al. 2020) and Veery (Halley et al. 2016). For example, Marshall (2001) described the Newfoundland Gray-cheeked Thrush as occurring in colonies, and in Siberia the average distance between nests of six individuals was 32 m (Kretchmar 1997).

The Gray-cheeked Thrush raises one brood per season. Clutches range from three to five eggs (typically four). The period from the time the first egg is laid until the young have naturally left the vicinity of the nest ranges occurs between late May and mid-July, depending on latitude (Rousseu and Drolet 2017). According to limited data, the Newfoundland Gray-cheeked Thrush typically breeds later in this period, with four nests reported, including two with eggs, on 26 and 30 June, and two with young on 9 and 13 July. Only the females are believed to incubate the eggs, for a period of 12 to 13 days (Baldwin 1955; Jehl and Hussell 1966) and the young likely leave the nest 11 to 13 days after hatching. Eight recently fledged juveniles were captured during passive mist netting in Newfoundland, on 14 to 18 August 2005 and 2006 (P. Taylor, I. Warkentin, and D. Whitaker, unpubl. data). No information is available on fledging rates.

Data from the Monitoring of Avian Productivity and Survivorship (MAPS) program from the Western Alaska and Northwestern Interior Forest Bird Conservation Regions (BCRs 2 and 4 respectively) yielded mean temporal and spatial estimates of apparent annual survival (that is the combined probability of surviving and returning to an area) for adult Northern Gray-cheeked Thrushes of 0.487 and 0.492, respectively (DeSante et al. 2015). Direct estimates of juvenile survival are not available, but mean temporal and spatial indices of productivity (an index of the annual number of young produced per adult) were 0.394 and 0.357, respectively (DeSante et al. 2015). The relevance of these rates to the Newfoundland Gray-cheeked Thrush is unknown. Adult survival during the breeding period is typically high for boreal songbirds (Whitaker et al. 2008), and predation by raptors is likely the key factor affecting adult survival of Gray-cheeked Thrushes during this period (Whitaker et al. 2020). Some nests are undoubtedly predated by American Martens (Martes americana) and Canada Jays, and predation by introduced Red Squirrels has likely reduced nestling and fledgling survival (Whitaker et al. 2015; McDermott 2021), as it has for Bicknell’s Thrush (Townsend et al. 2020; (see Current and Future Threats: Invasive and Other Problematic Species and Genes). Causes of mortality on migration routes and wintering grounds likely include predation by raptors and collisions with anthropogenic structures.

Habitat requirements

Breeding habitat

Historically, Newfoundland Gray-cheeked Thrush breeding habitat has consisted of windswept, coastal conifer forests and scrub, montane old-growth conifer forests, regenerating clear-cuts, and conifer scrub (Lamberton 1976a, b; Vassallo and Rice 1981; Thompson et al. 1999; Marshall 2001), although, in recent years, the subspecies’ occurrence in coastal conifer forests and scrub thickets on the Island of Newfoundland has been greatly reduced. However, the subspecies’ use of montane breeding habitat appears unchanged and, here, the Newfoundland Gray-cheeked Thrush is found in conifer scrub, regenerating harvested forest, mixed forest, and old-growth fir forest (Whitaker et al. 2015; McDermott et al. 2023). A common feature of all these vegetation types is high tree or shrub stem density.

Migration habitat

At migration stopovers, the Gray-cheeked Thrush generally occupies wooded areas with a dense understory. In the important spring stopover site in the Sierra Nevada de Santa Marta mountains in northeastern Colombia, migrating Gray-cheeked Thrushes occur in pre-montane forest at elevations of 1,000 to 2,000 m, shade coffee plantations, or lowland forest (Gómez et al. 2013, 2015; 2017). However, the Newfoundland subspecies likely only constitutes a small fraction of the total number of Gray-cheeked Thrushes observed (Gómez et al. 2019).

Winter habitat

Winter habitat use is undocumented for the Newfoundland Gray-cheeked Thrush, except for one GPS-tracked male that wintered on an approximately 1 ha territory in pre-montane tropical forest at an elevation of roughly 975 m in the Sierra Nevada de Santa Marta range in Colombia (Whitaker et al. 2018). A second individual was recorded at a single winter site in dense forest at 120 m ASL in Sierra de Perijá National Park, Venezuela, although details of the forest type used are unknown. Extensive deforestation and the conversion of pre-montane forest to shade coffee have occurred in the Sierra Nevada de Santa Marta range (Hansen et al. 2013); the latter habitat provides limited fruit as food for the Gray-cheeked Thrush (Bayly et al. 2016).

Movement, dispersal, and migration

Most Newfoundland Gray-cheeked Thrushes begin fall migration by 15 September (Whitaker et al. 2020; eBird 2022). Birds move south along the Atlantic Flyway, with most migration records coming from the Maritime provinces and the Atlantic coast of the United States (Figure 1). The birds then either cross the Gulf of Mexico to make landfall at the Yucatán Peninsula (or further south) and then travel overland through Central America to enter South America through the Darién region of Colombia or take a more direct route across the Caribbean to South America (Gómez et al. 2014; Whitaker et al. 2018, 2020). Birds crossing the Caribbean may stop over on islands such as Cuba (Figure 1; Aubry, unpublished data). Banding data from Darién, Colombia, indicate peaks in migration occur there from 8 to 30 October, with passage through the Sierra Nevada de Santa Marta mountains at a maximum in late October (Gómez et al. 2013, 2014). During the southbound fall migration, four individuals of the Newfoundland subspecies averaged 120 to 130 km/day (Whitaker et al. 2018).

The spring migration of the Gray-cheeked Thrush is more direct, with approximately two-week stopovers in the Sierra Nevada de Santa Marta mountains, before birds depart from 18 April to 21 May (peak 6 May) on non-stop northbound flights across the Caribbean to inland stopovers in the United States or Canada (range 870 to 3,500 km; Gómez et al. 2017). Individuals that cross the Gulf of Mexico rely on this key stopover site, and the rate of fuelling there may affect their fitness (Bayly et al. 2013; Gómez et al. 2014, 2017, 2019).

The northbound migration of the Gray-cheeked Thrush is rapid, and individuals return to Newfoundland beginning in the last week of May. During spring migratory flights, the minimal rate of travel was 40 to 75 km/h (960 to 1,800 km/day; Gómez et al. 2017); however, since these estimates do not include time at stopovers, the average rate of travel during the full journey would be much lower.

Little information is available on juvenile dispersal or site fidelity in the Newfoundland Gray-cheeked Thrush. During a tagging study in Newfoundland, 4 out of 29 adult males were recaptured in the same area in the following year (Whitaker et al. 2018) and another was recaptured less than 300 m away six years later (D. Whitaker and I. Warkentin, unpubl. data), indicating some breeding site fidelity.

Physiology

The Gray-cheeked Thrush has lower wing-loading than other Catharus thrushes, enabling it to use less energy to travel an equivalent distance and allowing shorter migration stopovers (Yong and Moore 1994). Individuals can accumulate sufficient fat reserves at a single stopover site to complete long-distance spring migration flights of greater than 3,000 km (Gómez et al. 2017). Failure to accumulate enough energy reserves to fuel long non-stop flights could increase migration time by up to 30 days, with important fitness consequences (Gómez et al. 2017).

Interspecific interactions

Diet

The Gray-cheeked Thrush forages on the ground and in the forest understory for invertebrates, including beetles, ants, wasps and bees, caterpillars, and spiders (Whitaker et al. 2020). Its diet shifts during fall and winter to include a large proportion of fruit, and fruit consumption is apparently higher during migration than on the breeding or wintering grounds (Gómez et al. 2018). Fruits eaten include those of forest trees of the families Melastomataceae, Rubiaceae, and Moraceae, as well as Toquilla Palm (Carludovica palmata; Blake and Loiselle 1992; Bayly et al. 2013; Gómez et al. 2014).

Predators and competitors

Little is known about predation of the Gray-cheeked Thrush. The only predation events that have been observed during breeding season involved the Peregrine Falcon (Falco peregrinus) in Alaska (Whitaker et al. 2020). Although peregrines do not nest in the breeding range of the Newfoundland Gray-cheeked Thrush, it is likely occasionally taken by other raptors, including Merlin (F. columbarius), Sharp-shinned Hawk (Accipiter striatus), and American Goshawk (A. atricapillus). Taverner and Swales (1907) reported Sharp-shinned Hawks taking Gray-cheeked Thrushes during fall migration in Ontario.

Indirect evidence indicates that Red Squirrels are a significant predator of Gray-cheeked Thrush eggs, nestlings, and recent fledglings (see Current and Future Threats: Invasive and Other Problematic Species and Genes). Whitaker et al. 2020), as is also the case with Bicknell’s Thrush (Townsend et al. 2020). The impacts of squirrel predation on the Newfoundland Gray-cheeked Thrush appear to be so severe that the two species rarely co-occur (Fitzgerald et al. 2017; McDermott 2021). The Red Squirrel was first successfully introduced to Newfoundland from Labrador in 1963 (at Main Brook and Roddickton; Whitaker 2015). In 1964, it was introduced to Camel Island in Notre Dame Bay, and then dispersed or was translocated to other nearby islands and the Island of Newfoundland by 1969. During the early 1970s, the squirrels were translocated to the Avalon Peninsula in eastern Newfoundland, Little Grand Lake in western Newfoundland, and La Poile River in southwest Newfoundland; other undocumented translocations are also suspected to have occurred. It spread from these disparate areas at a rate of around 5 km per year and was widespread by the mid-1980s and had colonized most suitable habitat on Newfoundland by the mid-1990s (Whitaker 2015). However, the incidence of Red Squirrels decreases with increasing elevation, and it is relatively uncommon above 300 m and very rare or absent above 500 m (McDermott et al. 2020). It has also been introduced or dispersed to many other nearshore islands around Newfoundland (for example, Change, Fogo, Thwart, and Swale islands).

There is no evidence of direct competition between the Gray-cheeked Thrush and other species during the breeding season. During migration and winter, Gray-cheeked Thrushes have been observed being displaced at ant swarms by Swainson’s Thrush (Catharus ustulatus) and several antbird species, and displacing Veery and Spotted Antbird (Hylophylax naevioides; Willis 1966).

Other interactions

The Gray-cheeked Thrush is susceptible to parasitism by Rabbit Ticks (Haemophysalis leporis-palustris) and Black-legged Ticks (Ixodes scapularis), as well as Chewing Louse (Myrsidea incerta) and flukes (Trematoda: Digenea; Whitaker et al. 2020). These interactions are unlikely to affect its survival or persistence.

The subspecies’ nesting habitat on Newfoundland and other islands may be altered or degraded by introduced herbivores, particularly Moose (Alces alces)—which is possibly exacerbated by Snowshoe Hares (Lepus americanus)—through excessive browsing that thins or kills the dense thickets and forest understories favoured by the thrushes (for example, Connor et al. 2000; Gosse et al. 2011; (see Current and Future Threats: Natural System Modifications).

Physiological, behavioural, and other adaptations

The Newfoundland Gray-cheeked Thrush seems to be able to accommodate some, but not all, human-induced changes to forest habitats. For example, during the breeding period, it makes extensive use of forests harvested by clear-cutting and strip-cutting, with its occurrence rate positively associated with the local extent of 12 to 18-year-old cutblocks harvested using either technique (Whitaker et al. 2015; McDermott 2021). However, in northern Colombia, the Gray-cheeked Thrush seems unable to adapt to the conversion of pre-montane forests to shade coffee plantations, where forage fruits are less available (Bayly et al. 2013, 2016). Thrushes in shade coffee plantations have a lower body mass, suggesting that they are less likely to successfully build up fat deposits for migration (Bayly et al. 2013, 2016), and may need to refuel more often or take a longer migratory route around the Gulf of Mexico, arriving on the breeding grounds up to 30 days later (Gómez et al. 2017).

Limiting factors

Limiting factors are generally not human induced and include intrinsic characteristics that make the species less likely to respond to conservation efforts. Limiting factors may become threats if they result in population decline. The main limiting factors on the Newfoundland Gray-cheeked Thrush are its largely insular breeding distribution, which makes it vulnerable to species introductions, and variable productivity and recruitment due to low survivorship and moderate population fluctuations. As a neotropical migrant, it is exposed to weather-related threats on its long migration pathway.

More than 90% of the breeding range of the Newfoundland Gray-cheeked Thrush is on islands, many of which have been subject to species introductions that irreversibly degrade the quality of its breeding habitat (Table 2; see also Current and Future Threats: Invasive and Other Problematic Species and Genes and Natural System Modifications). For example, introduced species on the Island of Newfoundland include nest predators such as the Red Squirrel and Eastern Chipmunk (Tamias striatus), as well as herbivores such as Moose and Snowshoe Hare, which can alter and degrade the dense thickets and forest understories favoured by thrushes (Dodds 1983). Many smaller nearshore islands in the subspecies’ breeding range have also been subject to these introductions (for example, Whitaker 2015). This insular distribution may be why the Newfoundland Gray-cheeked Thrush was historically more abundant and occurred in more southerly boreal ecosystems than the Northern Gray-cheeked Thrush.

The potential recovery of the Newfoundland Gray-cheeked Thrush population is likely constrained by a combination of low productivity and survivorship rates (see Life cycle and reproduction, above). Productivity rates for the Newfoundland Gray-cheeked Thrush are unknown, but those for the Northern Gray-cheeked Thrush are higher but more variable than those for four other Catharus species, perhaps linked to the more northerly breeding distribution of the Gray-cheeked Thrush (DeSante et al. 2015). However, this comparatively high productivity may be countered by low annual survival (DeSante et al. 2015). Breeding groups that fluctuate in size may be more susceptible to decline during, and thus are limited by, stochastic environmental events, such as adverse weather, during the breeding season. Newfoundland Gray-cheeked Thrush numbers may exhibit moderate fluctuations at two- or three-year intervals due to poor recruitment that reflects high squirrel numbers following Balsam Fir masting years (McDermott 2021; see Fluctuations and trends).

As a neotropical migrant, the Newfoundland Gray-cheeked Thrush is exposed to weather-related threats along its lengthy migration pathway, which likely includes extensive flights across the Caribbean Sea (Figure 1; Whitaker et al. 2018).

Population sizes and trends

Data sources, methods, and uncertainties

Although no single data set fully captures the status of the Newfoundland Gray‑cheeked Thrush, several citizen science projects and other types of surveys described here offer useful insights into population size and trends. Since the remaining population is now concentrated in areas that are difficult to access, including roadless montane forests and coastal islands, these areas are under-sampled. Consequently, targeted surveys similar to those developed for Bicknell’s Thrush (for example, Kouwenberg 2020) may be needed to better localize and monitor the population.

The North American breeding bird survey

The BBS is a continent-wide citizen science program that monitors trends in bird populations (Hudson et al. 2017; Sauer et al. 2017). The first BBS survey routes within the range of the Newfoundland Gray-cheeked Thrush were established on Newfoundland during the mid-1970s, and the network expanded to 25 routes during the early 1980s. By 2019, a total of 450 individual surveys had been completed along 34 BBS routes in the subspecies’ breeding range, yielding a data set that spans 46 years. The subspecies has been detected at least once on 26 (76%) of these routes, although detection rates have declined over time (Table 4).

Standardized BBS population and trend estimates for Canada are calculated using Bayesian hierarchical generalized additive models (Smith and Edwards 2020; Smith et al. 2020). Not all BBS surveys are included in trend calculations due to quality control criteria, and regional trends are only calculated based on provincial boundaries and BCRs. The analysis presented here was modified from the standard Canadian BBS analyses in two ways: (1) customized strata were created to generate trends specific to the breeding ranges of the two subspecies of the Gray-cheeked Thrush; and (2) many surveys in the range of the Newfoundland Gray-cheeked Thrush that had been removed using standardized filters (for example, due to starting too early or too late, or due to weather conditions) were added back in, increasing the number of surveys included in the analysis from 305 to 365. This latter step was important, as quality control filters had removed a disproportionate number of historical surveys, when the numbers of thrushes recorded were the highest (Table 4). With this approach, 25 of the 34 BBS routes surveyed in the Newfoundland Gray-cheeked Thrush’s range were used to estimate long-term (1974 to 2019; mean = 8.6 routes/year) and short-term (2009 to 2019; mean = 15.9 routes/year; Table 5) trends (Smith et al. 2020). The parameter estimates presented here are not subdivided among provinces, because no routes have been surveyed in the population’s range in Nova Scotia, and the only route in Quebec was established in 2019.

Although the BBS offers the most rigorous database for assessing population trends for the Newfoundland Gray-cheeked Thrush, it has limitations. First, the survey effort has varied over time, being most limited during the 1970s and 1990s, and the BBS route network was only expanded after 2010 to include portions of the subspecies’ range in southern Labrador and the Lower North Shore of eastern Quebec (Table 4). Second, BBS routes in the region generally follow roads connecting coastal communities, and therefore are disproportionately located in coastal areas where the Newfoundland Gray-cheeked Thrush has largely disappeared, and do not sample higher elevations and islands where the highest numbers occur today (Robineau-Charette et al. 2023).

Breeding bird atlases

Breeding bird atlases are large-scale citizen science projects that document the distribution and relative abundance of breeding birds at a provincial or regional scale. Atlas results are reported using a grid of 10 km x 10 km (100 km²) atlas squares. Surveys are primarily volunteer based, with skilled birders recording breeding evidence (that is, possible, probable, or confirmed breeding) for all bird species within each square. Recent atlas projects also incorporate point count surveys, which can be used to generate quantitative species occurrence or abundance estimates and are undertaken by skilled observers or through the use of Automated Recording Units (ARUs).

The first Quebec breeding bird atlas project ran from 1984 to 1989 (Gauthier and Aubry 1996) and the second one, from 2010 to 2014 (Robert et al. 2019), although the first atlas did not cover the Newfoundland Gray-cheeked Thrush’s range in that province. The first Newfoundland breeding bird atlas project began in 2020 and will end in 2024 (Newfoundland Breeding Bird Atlas 2023), although it does not include the portion of the subspecies’ range in Labrador. The three Maritime provinces have been covered in two atlas projects, the first from 1986 to 1990 (Erskine 1992), and the second from 2006 to 2010 (Stewart et al. 2015). All records attributed to the “Gray-cheeked Thrush” in the first Maritimes breeding bird atlas are presumed to have been Bicknell’s Thrush, and the islands where Newfoundland Gray-cheeked Thrushes were found during the second Maritimes atlas were not sampled during the first atlas (Whittam 2015). Many areas of the Newfoundland subspecies’ potential breeding habitat were under-sampled by the various atlas projects owing to difficulty of access, including the Lower North Shore of the Gulf of St. Lawrence, high-elevation areas and coastal islands (for example, Figure 3.1 in Robert et al. 2019). Because no areas used by the Newfoundland Gray-cheeked Thrush have been sampled in more than one atlas project, these data sets do not provide information on trends, although they offer insights into distribution and abundance.

Ebird

eBird is an online checklist program widely used by birders for reporting field observations (eBird 2022). The program was established in 2002, and its use has increased over time. Although the data are weighted to recent years, eBird records provide valuable information on recent distribution patterns and notable concentrations of individuals. This is particularly useful in the case of the Newfoundland Gray-cheeked Thrush, given the limitations of breeding bird atlases discussed above. In addition, the eBird coordinators for Newfoundland and Labrador have added reliable historical occurrence records from birdwatching and natural history reports and publications, including The Osprey (1970 to present; Nature Newfoundland and Labrador 2022), The Bullbird (1986 to 1990; Mactavish 1990), and the Nf. birds online birdwatching discussion group (1996 to present; Nf.birds 2022). Historical data from checklists from as far back as 1975 submitted to the Étude des populations d’oiseaux du Québec (ÉPOQ) database have also been added to eBird (ÉPOQ 2022).

Other data sources

Other studies offer insights into the population size of and population trends in the Newfoundland Gray-cheeked Thrush but have limited spatial and temporal scales. Avifaunal inventories of Gros Morne National Park in 1974 to 1975 (Lamberton 1976a) and in 1992 (Jacques Whitford Environment 1993) are of particular historical value, as are searches for the Gray-cheeked Thrush in western Newfoundland in 1981 to 1983 by Marshall (2001). In 2014 and 2015, Fitzgerald et al. (2017) surveyed historically occupied areas of Newfoundland and southern Labrador in an effort to understand the apparent decline of the Newfoundland Gray-cheeked Thrush. Surveys targeting the Newfoundland subspecies were also carried out in a study area in the Long Range Mountains in 2006 to 2007 and 2016 to 2017 (Whitaker et al. 2015; McDermott et al. 2023).

Abundance

No historical or contemporary population estimates have been published for the Newfoundland Gray-cheeked Thrush. The Boreal Avian Modelling Project (2020) generated a breeding season density map for the Gray-cheeked Thrush across its Canadian range using available point count data from 1991 to 2018, including those from the BBS and breeding bird atlas projects; the models used incorporated environmental predictors including vegetation, terrain, land use and climate. When applied to the breeding range of the Newfoundland Gray-cheeked Thrush, this yielded a median population estimate of 43,733 breeding males (90% CI 19,518 to 70,934). Although the sex ratio for the Gray-cheeked Thrush is unknown (Whitaker et al. 2020), presuming a 1:1 sex ratio would provide a population estimate of 87,466 mature individuals (90% CI 39,036 to 141,868) for the subspecies. However, it should be noted that the sex ratio for Bicknell’s Thrush (breeding adults) is typically about 2.5 males/female (Townsend et al. 2020).

Using BBS data from 2006 to 2015, Partners in Flight (2022) estimated that the Gray-cheeked Thrush population (both subspecies) in BCR 8 (Boreal Softwood Shield) in Newfoundland and Labrador totalled 54,000 individuals (95% CI 16,000 to 120,000). The boundaries of BCR 8 in Newfoundland and Labrador closely approximate the distribution of the Newfoundland Gray-cheeked Thrush in the province but take in an additional 37,411 km² (+30.9% of the range) in Labrador where the subspecies probably does not occur (Environment Canada 2013). If individuals are evenly distributed across BCR 8, this suggests a population estimate for the Newfoundland subspecies of about 41,245 mature individuals (95% CI 12,221 to 91,657).

For this assessment, the size of the population in each major biogeographic component of the subspecies’ breeding range was estimated based on the area occupied by the Newfoundland Gray-cheeked Thrush (Tables 3 and 6; (see Canadian range: Extent of occurrence and area of occupancy). The number of mature individuals was estimated by dividing the occupied area by the standard individual territory size, to estimate the total number of thrush territories. As there are no estimates of the size of individual Gray-cheeked Thrush territories, the territory size was approximated by using the maximum female territory size and minimum male territory size for Bicknell’s Thrush of 5.5 ha (Collins 2007). It was assumed that the number of mature individuals was twice the number of territories, based on a 1:1 sex ratio. This yielded an overall population estimate of about 144,182 mature Newfoundland Gray-cheeked Thrush individuals (95% CI: 112,907 to 188,529; Tables 3 and 6).

Breeding bird atlases provide information that may be used to estimate population size and distribution, although the dearth of records in the breeding range of the Newfoundland Gray-cheeked Thrush is limiting (Figure 3). Data submitted to the Newfoundland breeding bird atlas from 2020 to February 2023 represented 6,594 hours of survey effort in 687 atlas squares, and the Newfoundland Gray-cheeked Thrush was detected in just 20 squares (2.9%; Figure 3; Newfoundland Breeding Bird Atlas 2023). A portion of this survey time was spent completing 1,563 in-person point counts in 159 squares, yielding 14 detections (0.9% of point counts) of this species in six squares (3.8%; Newfoundland Breeding Bird Atlas 2023). However, montane areas and coastal islands, which likely support most of the current population, are under-represented in these results. The second Quebec atlas (2010 to 2014) covered the range of the Newfoundland Gray-cheeked Thrush in Quebec, including 741.2 hours of general atlassing effort in 21 squares and 50 point counts in nine squares in region 44 (Basse-Côte-Nord; Robert et al. 2019). However, no Gray-cheeked Thrushes were observed, suggesting that this species is at best very uncommon now along the Lower North Shore of Quebec. During the second Maritimes breeding bird atlas (2006 to 2010), Gray-cheeked Thrushes were only observed on three small islands along the Atlantic coast of Nova Scotia (Figure 3; Whittam 2015).

In summary, the Newfoundland Gray-cheeked Thrush is likely very rare in eastern Quebec, and the number of individuals in Nova Scotia is probably also very small due to the subspecies’ very restricted range there. Thus, most of Canada’s breeding population occurs in Newfoundland and Labrador, with most individuals breeding at high elevations on the Island of Newfoundland, along the north shore of the Strait of Belle Isle, and on coastal islands. Only the estimate of 144,182 mature individuals calculated for this report takes account of the strong effect of elevation on the subspecies’ distribution on the Island of Newfoundland (Figure 4; Table 3). However, this may overestimate numbers, as the occurrence rates used in that calculation came from targeted surveys of an area known to have robust numbers of thrushes. The average of three population estimates of about 91,000 mature individuals is therefore used here (Table 6).

^a See Population Sizes and Trends: Abundance and Table 3 for calculations.

^b See Boreal Avian Modelling Project (2020) and Population Sizes and Trends: Abundance for calculations.

^c See Partners in Flight (2022) and Population Sizes and Trends: Abundance for calculations.

^d 95% CI.

^e 90% CI.

Fluctuations and trends

Long-term historical trends

The annual rate of decline of the Newfoundland Gray-cheeked Thrush recorded by the BBS from 1974 to 2019 was -11.1% (95% CI: -14.5, -7.8) based on 25 routes (mean = 7.9 routes/year; Table 5; Figures 5 and 6; analyses completed following Smith and Edwards [2020]). This is among the steepest declines estimated by the BBS for any bird population in Canada and corresponds to an estimated cumulative decline from 1974 to 2019 of -99.5% (95% CI: -99.9, -97.5), with a 100% probability that Newfoundland Gray-cheeked Thrush numbers declined by at least -50% over that period.

As noted above, the BBS routes that coincide with the range of the Newfoundland Gray-cheeked Thrush do not sample higher elevations or nearshore islands. On the Island of Newfoundland, the subspecies has remained relatively more common in montane habitat, where squirrels are rare (see Canadian range and Current and Future Threats: Invasive and Other Problematic Species and Genes) but has likely also declined in these areas (Figure 4). If it is assumed that the subspecies’ abundance in habitats above 400 m has declined by -50%, based on Figure 4, and as these areas account for just 10% of the potential habitat (Table 3), this would imply an overall long-term decline of ‑95.5%. If it is assumed that the numbers on nearshore islands (2% of potential habitat) have also declined by only -50%, this would imply an overall decline of -93.5%. No attempt was made to rescale these estimates based on relative abundance, because there are no comparable historical data for high elevations and because the Newfoundland Gray-cheeked Thrush was relatively abundant at all elevations on Newfoundland in the 1970s and 1980s.

Figure 5. Rolling 10-year trend for changes in the Newfoundland Gray-cheeked Thrush population in Canada from 1984 through 2019, based on BBS data from 1974 to 2019 (n = 365 surveys of 25 routes; analysis based on Smith and Edwards [2020]). The orange and red horizontal lines show the thresholds for 10-year declines of 30% and 50%, which represent the COSEWIC thresholds for considering a species as Threatened and Endangered, respectively. Each point estimate represents the 10-year trend ending in a particular year (for example, the value reported for 2019 is the estimated trend for 2010 to 2019). The vertical bars represent 50% (dark blue) and 95% (light blue) credible intervals.

Figure 6. Annual index of population abundance for the Newfoundland Gray-cheeked Thrush in Canada, based on BBS data from 1974 to 2019 (n = 365 surveys on 25 routes; see also Table 5; analysis based on Smith and Edwards [2020]). The light-grey bars indicate the number of BBS surveys included in the analysis each year, and the dark grey dots indicate the mean number of thrushes observed on the surveys that year.

The long-term decline of the Newfoundland Gray-cheeked Thrush indicated by the BBS data is corroborated by other surveys and observations. For example, this subspecies is now absent or extremely rare at elevations below roughly 400 m throughout most of the Island of Newfoundland, where most of the historical population would have occurred (for example, Fitzgerald et al. 2017; eBird 2022; McDermott et al. 2023; see Canadian range and Abundance). Consistent with the temporal trend displayed by the BBS data (Figure 6), the earliest report of the subspecies’ decline is a 1992 avifaunal inventory of Gros Morne National Park, which noted an alarming drop in Gray-cheeked Thrush numbers in the park’s lowlands since the mid-1970s (Jacques Whitford Environment 1993).

Short-term trends

The annual rate of decline of the Newfoundland Gray-cheeked Thrush recorded by the BBS from 2009 to 2019 was -11.8% (95% CI: -21.2, -2.4), based on data from 25 routes (mean = 15.6 routes/year; Table 5 and Figure 7; analyses completed following Smith and Edwards [2020]). This corresponds to an estimated cumulative 10-year decline of ‑71.4% (95% CI: -90.7, -21.4), with a 95.7% probability that the Newfoundland Gray-cheeked Thrush declined in the areas surveyed by at least -30% over this period, and an 85.1% probability that it has declined by at least -50%. The rate of decline estimated from this analysis of BBS data for the five-year period from 2014 to 2019 was steeper (‑14.9% per year; 95% CI: -28.5, -2.1; see Figure 5) than in the previous five-year period, suggesting a cumulative 5-year decline of -55.4% (95% CI: -81.3%, -10.0%).

Again, the BBS routes that coincide with the range of the Newfoundland Gray-cheeked Thrush do not sample higher elevations. Whitaker et al. (2015) reported an encounter rate of 7.4% (119 of 1,613 survey points) across a 200‑km² survey area spanning an elevation range of 300 to 600 m in the Long Range Mountains of western Newfoundland, sampled in 2006 and 2007. In 2016 and 2017, McDermott et al. (2023) re-surveyed this same landscape and detected the Newfoundland Gray-cheeked Thrush at 14.9% of survey points (240 of 1,670 points). Although both surveys incorporated broadcasts of Gray-cheeked Thrush songs and calls, other aspects of the survey protocol differed,^{Footnote 2} likely increasing thrush detection probability in 2016 and 2017, so the detection rates are not directly comparable. Still, in contrast to the BBS trend, these studies suggest that thrush numbers in this montane landscape did not decline dramatically from 2006 to 2007 to 2016 to 2017.

Figure 7. Short-term (2009 to 2019) annual rate of population change for the Newfoundland Gray-cheeked Thrush estimated from BBS data (n = 172 surveys on 25 routes; see also Table 5; analysis based on Smith and Edwards [2020]). The larger light-grey dots indicate the number of BBS surveys included in the analysis for each year, and the small dark grey dots represent the mean number of thrushes observed on the surveys that year.

Although Newfoundland Gray-cheeked Thrush numbers have remained higher in montane areas, terrain higher than 400 m ASL accounts for just 10% of the overall breeding range (Table 3). Therefore, if it is assumed that the Newfoundland Gray-cheeked Thrush has not declined in these areas since 2009, but that occurrence was equal at all elevations in 2009, then it could be inferred that the overall population decline is just ‑64.3%. However, thrush abundance had already declined at low elevations in 2009 (SSAC 2010; Whitaker et al. 2015), so it is more realistic to re-scale these estimates based on an index of relative abundance between low and high elevation areas in 2009. The estimated encounter rate in the BBS in 2009 was 0.008 birds per stop, while the best comparable estimate for higher elevations comes from McDermott (2021), who calculated an encounter rate of 0.098 during the first four minutes of silent listening in point counts above 400 m (n = 1,189; J. McDermott unpubl. data). Multiplying the proportion of the subspecies’ range that is found above and below 400 m by these rates suggests that 42.6% of the population occurred at elevations below 400 m in 2009, resulting in an overall decline from 2009 to 2019 that may have been as low as -30.4%. If it is assumed that Newfoundland Gray-cheeked Thrush numbers have remained stable on the coastal islands, which represent 2% of the population’s breeding range (Table 3), and that individuals occur on these coastal islands at a similar rate to that in montane habitats, this would imply a 10-year decline of -26.7%. By applying these same approaches to the 2014 to 2019 period and by using the estimated encounter rate per stop derived from the BBS in 2014 (0.005 thrushes per stop), we calculate a net decline of -17.6% over five years, assuming that numbers remained stable at elevations greater than 400 m, and a net decline of ‑15.1% if it is also assumed that numbers remained stable on coastal islands where squirrels are not present.

Summary

BBS data document the near disappearance over the long term, in the areas surveyed, of the once abundant and widespread Newfoundland Gray-cheeked Thrush, a precipitous decline that has also continued over the past 10 years. This applies primarily to elevations below roughly 400 m on the Island of Newfoundland, an area that represents around 82% of the subspecies’ breeding range and would have supported the vast majority of the historical population. This pattern is corroborated by other studies and observations. Assuming that the abundance of the Newfoundland Gray-cheeked Thrush has been relatively stable in known and presumed refugial areas not covered by the BBS (that is, higher elevations and nearshore islands) since 2009, and accounting for the subspecies’ relative abundance at high and low elevations, the estimated overall population decline is -26.7% to ‑30.4% from 2009 to 2019. A similar analysis suggests a decline of -15.1% to -17.6% over the five years from 2014 to 2019. Finally, it is noteworthy that the long- and short-term trends for the Newfoundland Gray-cheeked Thrush are strikingly different from those for the Northern Gray-cheeked Thrush in Canada, which are generally neutral or positive (Table 5; Smith et al. 2020).

Population fluctuations, including extreme fluctuations

The Newfoundland Gray-cheeked Thrush is not known to exhibit extreme fluctuations in abundance of more than an order of magnitude. McDermott et al. (2023) suggested that, across a 257-km² study area, a drop in the detection rate at survey points from 11.8% to 10.3% from 2016 to 2017 may have resulted from poor recruitment in 2016, when Red Squirrel abundance was four times higher. Large fluctuations in squirrel abundance are linked to pulsed cone production by Balsam Fir (masting), which typically occurs at intervals of two or three years (Boutin et al. 2006; Robineau-Charette and Whitaker 2017a; McDermott et al. 2020). Similar population fluctuations caused by cyclical fluctuations in Red Squirrel abundance and nest predation have been reported for Bicknell’s Thrush (McFarland et al. 2008; Hill et al. 2019), Brown Creeper (Certhia americana; Poulin et al. 2010) and American Redstart (Sherry et al. 2015; see also Martin and Joron 2003). Consequently, it is reasonable to expect moderate fluctuations in Newfoundland Gray-cheeked Thrush abundance at two- or three-year intervals that result from poor recruitment following Balsam Fir masting years, in parts of the range where the Red Squirrel occurs.

Severe fragmentation

Given the predominance of islands in the breeding range of the Newfoundland Gray-cheeked Thrush, this range is inherently fragmented to some degree (Table 2; Figure 2). Although this phenomenon has not been specifically studied in the subspecies (Whitaker et al. 2020), the Newfoundland Gray-cheeked Thrush is likely well adapted to dispersal between isolated groups of breeding birds, for example, via extensive natal dispersal, as is the case for Bicknell’s Thrush (Studds et al. 2012; Townsend et al. 2020). However, the degree of isolation of individual breeding groups may have increased considerably since the 1970s due to the loss of approximately 93.5 to 95.5% of the overall population, in particular from much of coastal Newfoundland (see Population Sizes and Trends). These areas may now act as population sinks due to colonization by the Red Squirrel (see Current and Future Threats: Invasive and Other Problematic Species and Genes). The loss of this large, widespread reservoir of individuals has likely reduced the possibility of demographic rescue of remnant groups on smaller islands, as well as those occupying now-disjunct mountain massifs. Overall, the Newfoundland Gray-cheeked Thrush population is not considered to be severely fragmented, as most individuals are not found in very small or relatively isolated groups.

Rescue effect

The only portion of the breeding range of the Newfoundland Gray-cheeked Thrush that lies outside Canada is in the small French territory of Saint-Pierre-et-Miquelon, 20 km off the south coast of Newfoundland (Figure 2). This group of three islands and several smaller islets totalling around 242 km² is biogeographically part of the Newfoundland archipelago. Immigration from Saint-Pierre-et-Miquelon to Canada is biologically possible, and immigrants would be adapted to survive in Canada. However, although the Newfoundland Gray-cheeked Thrush was an abundant breeding bird in Saint-Pierre-et‑Miquelon in the 1970s, it is now considered very rare there (Etcheberry 1982; Etcheberry pers. comm. 2022; Jackman pers. comm. 2022), with only two individuals reported on Miquelon Island in the summer of 2023 (McDermott unpubl. data.). Causes of the decline in Saint-Pierre-et-Miquelon are uncertain, as squirrels were introduced but failed to persist (Whitaker 2015). The territory is thus unable to serve as a substantial source of immigrants, and there is no possibility of significant immigration or demographic rescue.

Threats

Historical, long-term, and continuing habitat trends

Extensive industrial forestry has taken place throughout much of the Newfoundland Gray-cheeked Thrush’s historical breeding range for over 100 years but has likely affected a smaller proportion of the landscape than natural forest disturbances have (Arsenault et al. 2016). In contrast, the decline of the Newfoundland Gray-cheeked Thrush has occurred very rapidly over the last 40 years or so. Furthermore, timber harvesting and silviculture have been limited at higher elevations and on coastal islands, where the largest segment of the thrush population persists today. In fact, much of the subspecies’ habitat is either unaffected by forestry (for example, conifer scrub) or is created by it (for example, regenerating clear-cuts; McDermott 2021), and the thrush has also declined in protected areas that lack large-scale forestry (for example, Jacques Whitford Environment 1993). Instead, species introductions are responsible for widespread habitat degradation on the breeding grounds. Intensive browsing by Moose and, to a lesser extent, Snowshoe Hare, can degrade or destroy the dense habitats preferred by the Newfoundland Gray-cheeked Thrush. Additionally, as forests change with post-harvest and post-disturbance succession, the habitat quality in higher-elevation areas may be reduced with further colonization by Red Squirrels. As a consequence, habitat quality has been degraded in a large part of Newfoundland and some other islands due to colonization by squirrels and other introduced species.

Extensive deforestation and conversion of pre-montane forest to shade coffee plantations and pastureland has occurred across the subspecies’ winter range, and may be accelerating in Colombia during the post-conflict period (Hansen et al. 2013; Zúñiga‑Upegui et al. 2019; Clerici et al. 2020; Gonzalez et al. 2023). The Sierra Nevada de Santa Marta mountains, also in Colombia, are likely an important wintering area for the Newfoundland Gray-cheeked Thrush, as well as providing key stopover sites on both spring and fall migration. Shade coffee production there has resulted in land-cover types that offer limited food compared to native forest, and are little used by Gray-cheeked Thrushes during migration (Bayly et al. 2013, 2016; Gómez et al. 2015). In addition, although the Sierra Nevada de Santa Marta range and Darién region represent two of the largest expanses of forest in northern Colombia, they are separated by a broad swath of land that has largely been converted to agriculture (Gómez et al. 2014). Consequently, thrushes migrating to northern South America via the Darién region potentially cross a 400‑km‑wide expanse of degraded habitat.

Current and future threats

The Newfoundland Gray-cheeked Thrush is vulnerable to the cumulative effects of various threats. These threats are categorized below and in Appendix 1, following the IUCN-CMP (International Union for the Conservation of Nature – Conservation Measures Partnership) unified threats classification system (based on Salafsky et al. 2008). The evaluation assesses the impacts of each of 11 main categories of threats and their subcategories, based on the scope (proportion of population exposed to the threat over the next 10-year period), severity (predicted population decline among those exposed to the threat, during the next 10 years or 3 generations, whichever is longer), and timing of each threat. The overall threat impact is calculated by taking into account the separate impacts of all threat categories, and can be adjusted by the species experts participating in the evaluation.

The overall threat impact on the Newfoundland Gray-cheeked Thrush is considered to be High - Medium, corresponding to an anticipated further decline of 3 to 70% over the next ten years (see Appendix 1 for details). Threats are discussed below in order of decreasing magnitude of impact (greatest to least), ending with those for which the scope or severity is unknown. Those threats with negligible threat impact are noted in Appendix 1, but are not discussed here.

IUCN 8, invasive and other problematic species and genes (medium impact)

Description of threat

According to several lines of evidence, invasive non-native species (IUCN 8.1), particularly the Red Squirrel, are the primary threat that precipitated the disappearance of the Newfoundland Gray-cheeked Thrush from much of its breeding range. These squirrels can severely impact thrush recruitment by depredating eggs, nestlings, and possibly fledglings, as is the case with Bicknell’s Thrush (McFarland et al. 2008; Hill et al. 2019; Townsend et al. 2020). The subspecies’ precipitous decline on Newfoundland from the 1970s through the 1990s (Figure 6) coincided with the rapid spread of squirrels across the island, which was accelerated by repeated translocations (see Interspecific interactions: Predators). For example, the Red Squirrel was first observed in Gros Morne National Park in 1975 (Minty 1976), when the Newfoundland Gray-cheeked Thrush was one of the most abundant bird species in the park’s extensive coastal lowlands (Lamberton 1976a). However, by 1992, the squirrels were common, while the thrush had virtually disappeared from this area (Jacques Whitford Environment 1993). Over this period, average annual counts of the subspecies along BBS route 57021 through the area fell from 24 birds per year in the mid-1970s (n = 2 surveys), to 7.5 birds per year from 1981 to 1984 (n = 4 surveys), and to one bird per year during the 1990s (n = 5 surveys); these thrushes have been absent from this route since 1997 (n = 10 surveys; Smith and Edwards 2020). Red Squirrels have now colonized most of the Island of Newfoundland, but decrease in abundance with increasing elevation, and are very rare or absent above 500 m (McDermott et al. 2020). They have also colonized many smaller coastal islands in the subspecies’ breeding range, although many others remain squirrel-free (Whitaker 2015). Data are limited, but all but one of the seven islands around Newfoundland where the subspecies is known to persist are squirrel-free, whereas the thrush appears to have disappeared from other islands colonized by squirrels (D. Whitaker, unpubl. data). The Newfoundland Gray-cheeked Thrush and Red Squirrel may now seldom co-occur, although there is overlap at elevations of roughly 350 to 500 m (12% of the breeding range), where both species are uncommon, but which may support about 33% of the current thrush population (Table 3; McDermott 2021; see also Fitzgerald et al. 2017). In this zone of overlap, Newfoundland Gray-cheeked Thrush numbers and reproductive success may vary at two- or three-year intervals due to wide cyclical variations in squirrel abundance that result from Balsam Fir masting cycles (see Fluctuations and trends: Population Fluctuations). The Red Squirrel’s colonization of the current suitable habitat on Newfoundland is likely largely complete, although conifer masting, forestry and other ecosystem changes may allow gradual continued squirrel expansion into thrush breeding areas at higher elevations. In addition, translocations and dispersal may allow squirrels to invade additional coastal islands.

No comprehensive data are available on relative densities of Red Squirrels across the range of the Northern Gray-cheeked Thrush that might help explain why that subspecies has persisted in areas where squirrels naturally occur. However, the co-occurrence of the Red Squirrel and the Northern Gray-cheeked Thrush may be relatively limited, as the southern portion of the subspecies’ range overlaps only slightly with the northern limit of the Red Squirrel’s range. Furthermore, these species may be segregated to some extent, as some southerly populations of the Northern Gray-cheeked Thrush are also restricted to montane forests (Höhn and Marklevitz 1974; Di Corrado 2015), while in other areas, this subspecies makes extensive use of deciduous thickets (Marshall 2001; Whitaker et al. 2020), which may be of limited habitat value to the Red Squirrel.

The Newfoundland Gray-cheeked Thrush is also likely affected by other species introduced to Newfoundland and other islands in its breeding range. The Eastern Chipmunk, which was introduced to Newfoundland in 1963 (Dodds 1983), is known to depredate bird eggs and young, and has colonized many productive habitats as well as towns and parks. This includes some areas that were historically occupied by the Newfoundland Gray-cheeked Thrush, such as southwestern Newfoundland. Although the overlap between these two species’ current distributions is limited, additional dispersal or translocations of chipmunks, for example to nearshore islands, could impact the Newfoundland Gray-cheeked Thrush.

No problematic native species or diseases (IUCN 8.2) are known to be threats to the Newfoundland Gray-cheeked Thrush. However, high-elevation areas have typically been free of large-scale disturbance, leading to the establishment of old-growth fir stands, which still support many breeding thrushes (McCarthy and Weetman 2006; Whitaker et al. 2015). An Eastern Spruce Budworm (Choristoneura fumiferana) outbreak is currently developing in western Newfoundland, including these montane fir forests (Government of Newfoundland and Labrador 2022a). It is unknown whether this will progress to a stand-killing outbreak at high elevations, and a budworm control program is underway. However, forest management practices and selective browsing of young hardwoods by introduced Moose have helped create landscapes dominated by even-aged, conifer-dominated stands, and this as well as climate change may favour more extensive and severe budworm outbreaks than would occur naturally (for example, Blais 1983; Régnière et al. 2012; Robert et al. 2012, 2018; Pureswaran et al. 2015). Widespread defoliation would likely lead to short-term dietary benefit to the Newfoundland Gray-cheeked Thrush, as boreal songbirds take advantage of an abundance of budworm caterpillars to feed nestlings and fledglings, and many species experience high recruitment and local or large-scale population growth as a result (Holmes et al. 2009; Venier et al. 2009; Venier and Holmes 2010; Walker and Taylor 2020). However, these dietary benefits may be offset by impacts to habitat (see IUCN 7, Natural System Modifications).

Scope: restricted (11 to 30%)

The threat from Red Squirrel colonization (IUCN 8.1) is most relevant to the Newfoundland subspecies on nearshore islands where squirrels do not yet occur (Payne 1976), and to thrushes at intermediate or high elevations if conifer masting, forestry activities, climate change, and other ecosystem changes allow the squirrel population to expand into these areas. If the budworm outbreak (IUCN 8.2) progresses into high-elevation forests over the next 10 years, this may provide benefits through short-term increased food availability.

Severity: serious (31 to 70%)

The Newfoundland Gray-cheeked Thrush has disappeared from virtually all areas where Red Squirrels have become abundant, and it seems likely that any expansion of the squirrel’s range will seriously impact or locally extirpate the thrush. These impacts could be exacerbated by other invasive or problematic species.

IUCN 2, agriculture and aquaculture (low impact)

Description of threat

Production of annual and perennial non-timber crops (IUCN 2.1) and livestock farming and ranching (IUCN 2.3) may cause loss or degradation of habitats in northern South America that provide key wintering and migratory stopover habitat for the Newfoundland Gray-cheeked Thrush (Gómez et al. 2014, 2017, 2019; Whitaker et al. 2018). This region is experiencing continued extensive deforestation (Hansen et al. 2013; Gonzalez et al. 2023), which may be accelerating in the post-conflict period in Colombia (Zúñiga-Upegui et al. 2019; Clerici et al. 2020). This includes the clearing of forests for pastureland, a habitat that is not used by the Newfoundland Gray-cheeked Thrush, as well as the conversion of pre-montane forest to shade coffee plantations, which offer limited food (that is, fruit) compared to native forests and are little used by the Gray-cheeked Thrush (Bayly et al. 2013, 2016; Gómez et al. 2015). Kramer et al. (2018) found that Golden-winged Warbler (Vermivora chrysoptera) breeding populations that had high migratory connectivity to a restricted wintering area in the same region were declining, while breeding populations that wintered elsewhere remained stable. However, given the reduced size of the Newfoundland Gray-cheeked Thrush population, the availability of winter habitat may not be a limiting factor. While pastureland and shade coffee are of limited habitat value, at least 38% of the land at elevations of 1,000 to 2,300 m across the Colombian Andes still has valuable forest cover, with higher proportions in the northern Andes (Gonzalez et al. 2023), which overlaps with the suspected winter range of the Newfoundland subspecies. The breeding areas that now appear to be used by the Newfoundland Gray-cheeked Thrush are poorly suited to agricultural development, and will be largely unaffected by this threat.

Scope: restricted - small (1 to 30%)

Conversion of pre-montane forest, including that in protected areas, to shade coffee plantations (IUCN 2.1) or for livestock grazing (IUCN 2.3), has been extensive across Colombia and may have increased in recent years (Zúñiga-Upegui et al. 2019; Clerici et al. 2020; Gonzalez et al. 2023). These activities are likely to continue over the next 10 years in the areas where most Newfoundland Gray-cheeked Thrushes are suspected to winter.

Severity: slight (1 to 10%):

Conversion of forest to pastureland or annual crops leads to an outright loss of potential habitat, causing the displacement of thrushes, while conversion to shade coffee plantations reduces habitat quality. However, given the reduced size of the Newfoundland Gray-cheeked Thrush population, the availability of winter habitat may not be an important limiting factor.

IUCN 3, energy production and mining (low impact)

Description of threat

Some mining and quarry development (IUCN 3.2) is expected in the breeding range of the Newfoundland Gray-cheeked Thrush, including the Valentine gold mine project in central Newfoundland (project area 240 km²; Marathon Gold 2019). The project area includes higher elevations (320 to 480 m) where Newfoundland Gray-cheeked Thrushes have been observed (Marathon Gold 2020). Additional surface and subsurface mines are under development on the Baie Verte Peninsula (Anaconda and Rambler mines), in areas where the Newfoundland Gray-cheeked Thrush likely occurred in the past (Government of Newfoundland and Labrador 2022b).

A moratorium on wind-energy development (IUCN 3.3) in Newfoundland and Labrador was removed in 2022. This was followed by a call for nominations of areas of interest, which resulted in 31 submissions, and in December 2022 the province issued a call for bids for wind energy projects on roughly 16,600 km² of crown land on Newfoundland (> 15% of the island), which attracted bids from 19 companies. An existing proposal in western Newfoundland (World Energy GH2 2022) is being reviewed; it involves an initial 164-turbine, 135-km² wind farm (maximum elevation 350 m) on the Port-au-Port Peninsula, to be operational by 2024. Two additional sites are slated for similarly scaled development by 2025: the Lewis Hills and Blow-me-down Mountains (maximum elevation 814 m), and the Anguille Mountains (maximum elevation 500 m), both of which have extensive suitable breeding habitat for the subspecies.

Oil and gas drilling (IUCN 3.1) is almost non-existent in the breeding range of the Newfoundland Gray-cheeked Thrush. With regard to its winter range, Venezuela is one of the world’s largest oil producers, although most extraction occurs in Lake Maracaibo and along the Orinoco Belt, east of the subspecies’ suspected winter range.

Scope: small (1 to 10%)

The total area of mining projects (IUCN 3.2) and the area involved in the initial phase of wind farm development (IUCN 3.3) are limited, and the footprint of wind energy projects is typically confined to localized clearings for turbine pads, access roads, and transmission lines. However, the montane and coastal/island habitats where the subspecies remains more common typically have a high potential for wind energy, and the 2022 call for proposals in Newfoundland and Labrador could lead to a surge in development over the next 10 years. The amount of thrush habitat that could be affected over the same period by other types of development such as quarries is relatively limited.

Severity: moderate - slight (1 to 30%)

Site development for surface mines and quarries (IUCN 3.2) and wind turbines (IUCN 3.3), as well as supporting infrastructure such as access roads and transmission lines, requires the removal of all vegetation, leading to the destruction of all thrush habitat within the project footprint, as well as the potential for some mortality from direct strikes with turbine towers or blades. In addition, some impacts may occur in the surrounding residual habitat (for example, industrial noise including blasting, ore crushing, and turbine operation), while mortality from wind turbine strikes may occur during migration.

IUCN 5, Biological resource use (low impact)

Description of threat

Although the clear-cutting of montane forests occurred in the subspecies’ breeding areas during the 1990s and early 2000s (McDermott 2021), little or no commercial logging or wood harvesting (IUCN 5.3) is expected in these areas over the next 10 years. Clear‑cuts offer little cover in the short term, but are regularly used by thrushes at roughly 10 to 20+ years post-harvest. However, 30 to 80-year-old stands may be of reduced habitat value (Thompson et al. 1999; Whitaker et al. 2015; McDermott 2021), so stands cut during the 1990s and early 2000s will likely be of declining habitat value over the next 10 years. These second-growth stands in montane forests may also support more Red Squirrels (McDermott et al. 2021), with concomitant increases in nest predation (see IUCN 8.1, Invasive Non-Native/Alien Species/Diseases). Pre-commercial thinning of regenerating stands, which reduces habitat quality for Bicknell’s Thrush (Chisholm and Leonard 2008; Aubry et al. 2011), is predicted to have similar impacts on Gray-cheeked Thrush habitat (Whitaker et al. 2015). Pre-commercial thinning was widely carried out in Newfoundland during the 1990s, but is now rarely used (Government of Newfoundland and Labrador 2014). Timber harvesting also occurs in the winter range and may be increasing in Colombia in the post-conflict period, although the overall impact on Newfoundland Gray-cheeked Thrush habitat may be limited over the next 10 years (Hansen et al. 2013; Clerici et al. 2020).

Scope: small (1 to 10%)

Logging (IUCN 5.3) on the subspecies’ breeding grounds during the next 10 years is likely to be limited to small-scale domestic wood harvesting. However, more than 50 km² of clear‑cuts created during the 1990s and early 2000s in Newfoundland will decline in value as Newfoundland Gray-cheeked Thrush habitat, and may increase in value as Red Squirrel habitat. Some timber harvesting also occurs in the subspecies’ winter habitat and may be increasing.

Severity: moderate (11 to 30%)

Declining habitat quality for the Newfoundland Gray-cheeked Thrush and increasing habitat quality for the Red Squirrel is expected to occur gradually as existing clear-cuts mature into second-growth forest (IUCN 5.3). The degree to which habitat availability is limiting during winter is unknown.

IUCN 7, natural system modifications (low impact)

Description of threat

Disruption caused by outbreaks of defoliating insects (that is, Eastern Spruce Budworm and Hemlock Looper [Lambdina fiscellaria]; IUCN 7.3) could affect the Newfoundland Gray-cheeked Thrush. Under Newfoundland and Labrador’s Early Intervention Strategy Program (early intervention strategy, or EIS; Johns et al. 2019; Régnière et al. 2019), the bacterial pesticide Btk is used to prevent budworm outbreaks from developing in western Newfoundland. While Btk is not considered toxic to birds (Johns et al. 2019; MacLean et al. 2019), its application reduces food availability for insectivores by killing caterpillars, and forest insect control programs may reduce reproductive success and dampen the population responses of insectivorous birds (Holmes 1998; Norton et al. 2001; Awkerman et al. 2011; Drever et al. 2018). It is uncertain how budworm control affects the Gray-cheeked Thrush, and the prevention of an outbreak may have mixed positive and negative effects over the short and long term (see IUCN 8.2: Problematic Native Species/Diseases). Budworm outbreaks may result in large areas of old-growth forest being converted to young, regenerating stands, with varied consequences for the Newfoundland Gray-cheeked Thrush over time. During the outbreak, thrushes likely benefit from an abundance of caterpillars to feed nestlings. The resulting 10 to 20 year old or more regenerating stands could then serve as breeding habitat. However, recently disturbed stands (< 10 years old) and older regenerating stands (40 to 80 years post-disturbance) may have poorer habitat quality, and 40 to 80 year old stands may also have increased cone production, allowing squirrels to expand upslope into thrush habitat in the future (McDermott et al. 2020; McDermott 2021).

Introduced Moose on the Island of Newfoundland lack natural predators, and can become hyperabundant in places where hunting pressure is limited, leading to the severe over-browsing of woody vegetation up to 4 m high (Connor et al. 2000; McLaren et al. 2000). This can cause the degradation or destruction of dense regenerating forests and shrub thickets, as well as opening up the understories of mature fir forests (McLaren et al. 2004; Gosse et al. 2011; Robineau-Charette and Whitaker 2017b; see also Smith et al. 2010), all of which serve as breeding habitat for the Newfoundland Gray-cheeked Thrush. Forest degradation and loss due to over-browsing became so severe in Gros Morne and Terra Nova national parks that they were opened to Moose hunting in 2011 (Gosse 2019; Robineau-Charette et al. 2021). As a result, the vegetation now shows signs of recovery, although many severely degraded stands remain, especially in areas with limited hunter access (Gosse 2019; Robineau-Charette et al. 2021). Browsing by introduced Snowshoe Hare may also affect the dense thickets and forest understories used by breeding thrushes, although probably to a much lesser degree than Moose (Dodds 1960, 1983).

Wildfire is rare in the wet coastal and montane boreal forests where Newfoundland Gray-cheeked Thrushes breed (Arsenault et al. 2016), so wildfire and fire suppression (IUCN 7.1) would have negligible consequences on this population. Although several large hydroelectric dams and reservoirs (IUCN 7.2) have been built in the subspecies’ breeding range, there are no plans to construct any major new dams or reservoirs in the next 10 years.

Scope: large (31 to 70%)

Under the EIS program, roughly 3,400 km² of western Newfoundland forest was sprayed with Btk in 2021 and 2022, including high-elevation areas known to support breeding Newfoundland Gray-cheeked Thrushes (Government of Newfoundland and Labrador 2022a). In areas of Newfoundland with limited road access and largely inaccessible to hunters, including large areas of montane forest, the Newfoundland Gray-cheeked Thrush is threatened by over-browsing by Moose (IUCN 8.1; McLaren et al. 2004; Gosse et al. 2011).

Severity: slight (1 to 10%)

The impacts of budworm prevention efforts on the subspecies’ reproductive success are speculative, based on their effects on other insectivorous birds, and may have a mix of positive and negative consequences on thrush habitat over the next 10 years. Degradation of thrush habitat by introduced herbivores is ongoing.

IUCN 11, climate change and severe weather (unknown impact)

Description of threat

Historically, montane forests on Newfoundland have not experienced major disturbances, enabling old-growth fir stands, which support breeding thrushes, to develop (McCarthy and Weetman 2006; Whitaker et al. 2015). However, a spruce budworm outbreak is developing in western Newfoundland, including the Long Range Mountains, potentially due in part to climate change (IUCN 11.1; see IUCN 8.2, Problematic Native Species/Diseases). Climate change is predicted to exacerbate budworm outbreaks at high elevations and latitudes, through the direct impacts of weather factors on the growth and survival of larvae (for example, Régnière et al. 2012), and by increasing the synchrony between budworm emergence and host tree budburst (Pureswaran et al. 2015; Bellemin-Noël et al. 2021). A severe outbreak could have mixed consequences for the Newfoundland Gray-cheeked Thrush and its habitat over the next 10 years and in the longer term. By increasing the extent of mid-successional forests, which produce more cones and are preferred by Red Squirrels (McDermott et al. 2020), this could also eventually enable the squirrels’ upslope expansion, with serious implications on recruitment in the Newfoundland Gray-cheeked Thrush (see IUCN 8.1, Invasive Non-Native/Alien Species/Diseases).

Increased frequency and intensity of storms (IUCN 11.4) throughout the year, and in particular, of tropical storms over the Caribbean Sea and Gulf of Mexico (Knutson et al. 2010; Walsh et al. 2016), may increase mortality of the subspecies during migration (Wiley and Wunderle 1993; Butler 2000).

Scope: large (31 to 70%)

The effects of the changing climate are anticipated to affect much of the Newfoundland Gray-cheeked Thrush’s breeding, migration and winter ranges.

Severity: unknown

It is unknown whether a budworm outbreak will occur in Newfoundland’s montane forests, or whether it will be hampered by annual weather effects or control programs. Although the frequency of severe weather events is expected to increase with climate change, the resulting impacts on the Newfoundland Gray-cheeked Thrush will be variable and difficult to predict over the next 10 years.

Number of threat locations

The Newfoundland Gray-cheeked Thrush has a naturally fragmented breeding range, and groups breeding in different areas are likely connected through dispersal movements. Most known threats are unlikely to affect individuals in these different areas at the same time. The introduced Red Squirrel is considered to be the most serious single plausible threat that could rapidly affect Newfoundland Gray-cheeked Thrush individuals, and Red Squirrels and other invasive species would typically have independent effects on isolated locations such as islands. It is assumed that colonization of any such location by species such as the Red Squirrel is likely to result in the rapid decline or local extirpation of the Newfoundland Gray-cheeked Thrush.

Assessments of the number of regional clusters of Newfoundland Gray-cheeked Thrush observations since 2009 indicate that the population is distributed across southern Labrador, on more than 10 coastal islands, and in 5 to 7 nesting aggregations on the Island of Newfoundland itself (Figure 2), with a wide scattering of isolated observations across Newfoundland. The breeding range of the Newfoundland Gray-cheeked Thrush also encompasses over 2,000 islands (Table 2), including 24 islands larger than 20 km². Most of these islands have not been surveyed, and it is likely that some support relatively large groups of breeding thrushes that have not yet been identified. As each island or isolated nesting aggregation occupied by the Newfoundland Gray-cheeked Thrush is considered a separate location, the number of locations is therefore likely significantly greater than 10.

Protection, status, and ranks

Legal protection and status

The Newfoundland Gray-cheeked Thrush is not listed in Canada under Schedule 1 of the Species at Risk Act, although the species and its nests are protected in Canada under the Migratory Birds Convention Act, 1994. The Newfoundland Gray-cheeked Thrush was listed as Vulnerable under the Newfoundland and Labrador Endangered Species Act in 2005, and was uplisted to Threatened in 2014 (SSAC 2010). It is not listed in Quebec under the Act Respecting Threatened or Vulnerable Species (R.S.Q., c. E‑12.01) and is not on the list of wildlife species likely to be designated threatened or vulnerable in the province. In addition, it is not listed in Nova Scotia under the Nova Scotia Endangered Species Act (S.N.S. 1998, c. 11).

Non-legal status and ranks

The Gray-cheeked Thrush (all subspecies) is ranked globally as G5 (Secure; NatureServe 2022), nationally as N5B (Secure Breeder; Canadian Endangered Species Conservation Council 2020; NatureServe 2022). Globally, the subspecies minimus is ranked G5T4 (Apparently Secure, last assessed in 2016). It is not ranked in Quebec or Nova Scotia, and is Unranked on the Island of Newfoundland (SNR) and in Canada (NNR) (NatureServe 2022). However, on the Island of Newfoundland, where only the minimus subspecies is present, the full species (Gray-cheeked Thrush) is ranked as S2B,SUM (Imperiled Breeder, Unranked Migrant).

Partners in Flight (2021) assessed the Gray-cheeked Thrush in BCR 8, which closely approximates the range of the Newfoundland Gray-cheeked Thrush, as having a regional combined score (breeding season) of 10/25 (mid-concern).

Land tenure and ownership

Since 2009, the Newfoundland Gray-cheeked Thrush has been recorded in one Canadian national park, two national historic sites, two provincial ecological reserves, and three provincial parks (Table 7). These areas protect 2,086 km² and encompass ∼1.7% of the subspecies’ breeding range. Of 15 non-breeding sites identified from movements of four GPS-tagged male individuals of the subspecies, three were in protected areas outside of Canada: La Güira National Park, Cuba, and Río Plátano Biosphere Reserve, Honduras, during fall migration, and Sierra de Perijá National Park, Venezuela, in winter (Whitaker et al. 2018).

Information sources

References cited

Allen, J.A. 1900. List of birds collected in the district of Santa Marta, Colombia, by Mr. Herbert H. Smith. Bulletin of the American Museum of Natural History 13:117-184.

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Collections examined

No collections were examined for the preparation of this report.

Authorities contacted

• Blaney, S. Executive Director/Senior Scientist. Atlantic Canada Conservation Data Centre. Sackville, New Brunswick
• Campbell, G. Wildlife Biologist. Canadian Wildlife Service, Environment and Climate Change Canada. Mount Pearl, Newfoundland and Labrador
• de Forest, L. Species Conservation Specialist. Species Conservation and Management, Parks Canada Agency. Halifax, Nova Scotia. COSEWIC member
• Durocher, A. Associate Data Manager. Newfoundland and Labrador Department of Fisheries, Forestry and Agriculture, Atlantic Canada Conservation Data Centre. Corner Brook, Newfoundland and Labrador
• Filion, A. Scientific and GIS Project Officer. COSEWIC Secretariat, Canadian Wildlife Service, Environment and Climate Change Canada. Gatineau, Quebec
• Gauthier, I. Biologiste, Coordonnatrice provinciale des espèces fauniques menacées ou vulnérables. Ministère des Forêts, de la Faune et des Parcs. Quebec City, Quebec. COSEWIC member
• Gonzalez-Prieto, A., Avian Landscape Ecologist. Wildlife Research Division, Science and Technology Branch, Environment and Climate Change Canada. Delta, British Columbia
• Humber, J. Ecosystem Management Ecologist, Biodiversity. Newfoundland and Labrador Department of Environment and Conservation. Corner Brook, Newfoundland and Labrador. COSEWIC member
• Klymko, J. Zoologist. Atlantic Canada Conservation Data Centre, and member of COSEWIC. Sackville, New Brunswick
• Leonard, T. Natural Areas Ecologist. Newfoundland Department of Environment and Climate Change. Corner Brook, Newfoundland and Labrador
• McDonald, R. Senior Environmental Advisor. Department of National Defence. Ottawa, Ontario
• Moores, S. Senior Manager, Wildlife Research. Newfoundland and Labrador Department of Environment and Conservation. Corner Brook, Newfoundland and Labrador. COSEWIC member
• Paquet, J. Shorebird Biologist. Canadian Wildlife Service, Environment and Climate Change Canada. Sackville, New Brunswick
• Rodrigues, B. Ecosystem Management Ecologist, Furbearers. Fisheries, Forestry and Agriculture. Corner Brook, Newfoundland and Labrador
• Schnobb, S. COSEWIC Secretariat, Canadian Wildlife Service, Environment and Climate Change Canada. Gatineau, Quebec
• Shepherd, P. Ecosystem Scientist III. Species Conservation and Management, Parks Canada Agency. Vancouver, British Columbia. COSEWIC member
• Smith, A. Senior Biostatistician. Canadian Wildlife Service, Environment and Climate Change Canada, National Wildlife Research Centre. Ottawa, Ontario
• St-Laurent, K. Species at Risk Recovery Biologist. Canadian Wildlife Service, Environment and Climate Change Canada. Sackville, New Brunswick
• Taylor, S. Weaver Brothers Distinguished Professor. School of Renewable Natural Resources, Louisiana State University. Baton Rouge, Louisiana. COSEWIC member
• Whittam, R. Head - Terrestrial and Marine Conservation. Canadian Wildlife Service, Environment and Climate Change Canada. Sackville, New Brunswick
• Wu, J. Scientific Project Officer ATK. COSEWIC Secretariat, Canadian Wildlife Service, Environment and Climate Change Canada. Gatineau, Quebec

Acknowledgements

Funding for the preparation of this report was provided by Environment and Climate Change Canada. The authorities listed below provided valuable data and/or advice. Thanks are extended by the report writers to Adam Durocher (Atlantic Canada Conservation Data Centre) for assistance with spatial data processing; Diana Stralberg (Natural Resources Canada) for providing an estimate of the Newfoundland Gray-cheeked Thrush population size on behalf of the Boreal Avian Modelling Project; Sandra Marquez (Birds Canada) for preparing the atlas coverage map; Greg Campbell and Becky Whittam (both of Environment and Climate Change Canada) for details on Gray-cheeked Thrush observations in Nova Scotia during the Maritime Breeding Bird Atlas; Adam Smith (Environment and Climate Change Canada) for advice on analysis of BBS population trends; Alain Filion (Environment and Climate Change Canada) for help with delineation of the area of occupancy; and Birds Canada for providing access to Newfoundland Breeding Bird Atlas data, as well as to all of the volunteer participants who gathered data for the project. The report writers also extend their thanks to Richard Elliot, COSEWIC Birds Specialist Sub-committee (SSC) co-chair, for his support and guidance in the preparation of this assessment, and to SSC members Christian Artuso, Andrew Horn, Jean-Pierre Savard, and David Toews for their review of earlier drafts.

Biographical summary of report writers

Darroch Whitaker completed an M.Sc. at Memorial University of Newfoundland in 1997, where he studied the conservation of riparian songbirds in Newfoundland’s managed forests, and a Ph.D. at Virginia Tech in 2003, where he studied the habitat ecology of Ruffed Grouse (Bonasa umbellus). Since 2007 he has worked as an ecologist with Parks Canada, where he has helped lead ecological integrity monitoring, species at risk, and ecological restoration programs in Gros Morne National Park and Torngat Mountains National Park. He is an adjunct professor at Memorial University of Newfoundland – Grenfell Campus. Over the past decade he has collaborated on a series of studies aimed at understanding the causes of the decline of the Gray-cheeked Thrush on Newfoundland and generating information to support recovery planning.

Jenna McDermott completed an M.Sc. in 2021 at Memorial University of Newfoundland, where she studied the habitat associations and distribution of the Gray-cheeked Thrush and Red Squirrel in western Newfoundland. She has been involved with Gray-cheeked Thrush research in Newfoundland and Labrador since 2014, including studies on the genetic differentiation of the Newfoundland and Northern Gray-cheeked Thrushes, and the migratory connectivity of the Newfoundland Gray-cheeked Thrush. Since 2020, she has worked for Birds Canada as the assistant coordinator of the Newfoundland Breeding Bird Atlas, conducting bird surveys across the Island of Newfoundland and providing resources and training for volunteers, among other tasks.

Ian Warkentin completed a Ph.D. at the University of Saskatchewan, studying the winter ecology of urban Merlins (Falco columbarius), and has been a professor in the Environmental Science Program at Memorial University - Grenfell Campus since 1994. He began studying songbirds in western Newfoundland boreal forests in 1997, examining the breeding biology and nesting habitat of the Northern Waterthrush (Parkesia noveboracensis) in the context of forest harvesting. He subsequently worked with Phil Taylor in western Newfoundland on forest songbird community ecology and the spatial ecology of focal species, including the Northern Waterthrush, Blackpoll Warbler (Setophaga striata), and Yellow-rumped Warbler (S. coronata). His collaborative work on the Newfoundland Gray-cheeked Thrush focuses on habitat affiliations and threats linked to the introduced Red Squirrel, as well as the migratory connectivity of individuals breeding in western Newfoundland forests.

Appendix - threats calculator

Species or ecosystem scientific name: Gray-cheeked Thrush minimus subspecies (Catharus minimus minimus)

Date: 2023-02-27

Assessor(s): Jenny Heron (facilitator); Darroch Whitaker, Jenna McDermott, and Ian Warkentin (report writers); Richard Elliot (Birds SSC co-chair), Claire Murley (Secretariat), Christian Artuso, Louise Blight, Mike Burrell, Pete Davidson, Bruno Drolet, John Gosse, Kevin Kardynal, Holly Lightfoot, Kathy St-Laurent, Steven Matsouka, Peter Thomas

References: Draft Gray-cheeked Thrush minimus status report (October 2022) and draft threats calculator

Assigned overall threat impact: BC = High - Medium

Impact adjustment reasons: Overall threat impact adjusted downwards by threat experts, due to lack of quantitative information on several threats, and potential overlap of individual level-2 scores.

Overall threat comments: Generation time for the Gray-cheeked Thrush minimus is approximately 2.31 years (Bird et al. 2020), so the timeframe for considering severity and timing is 10 years. Threats are considered on the breeding grounds in QC, NS, and NL; on migration through the eastern United States, the Caribbean and Central America; and on the wintering grounds in northern South America.

Classification of Threats adopted from IUCN-CMP, Salafsky et al. (2008).