Canada Warbler (Cardellina canadensis): COSEWIC assessment and status report 2020

Official title: COSEWIC Assessment and Status Report on the Canada Warbler Cardellina canadensis in Canada 2020

Committee on the status of Endangered Wildlife in Canada (COSEWIC)
Special concern 2020

COSEWIC assessment summary

Assessment summary – November 2020

Common name: Canada Warbler

Scientific name: Cardellina canadensis

Status: Special Concern

Reason for designation: This small songbird has 80% of its breeding range in Canada and winters in the northern Andes Mountains. Breeding Bird Survey results show that the long-term decline of the Canadian population began to slow down in 2003 and that numbers have increased steadily since 2012, with an overall growth of 46% over the past decade. However, significant threats persist, most notably clearing of forests in South America for livestock farming and other agriculture. The revised status reflects the substantial improvement in population trend since the previous assessment of Threatened, but concern remains that the species is at risk of becoming Threatened again if threats are not managed effectively.

Occurrence: British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, Yukon, Northwest Territories

Status history: Designated Threatened in April 2008. Status re-examined and designated Special Concern in November 2020.

COSEWIC executive summary

Canada Warbler
Cardellina canadensis

Wildlife species description and significance

Canada Warbler is a small, colourful songbird. Males are more brightly marked than females and immatures, with blue-grey upperparts and tail contrasting with a yellow throat and breast. Black stripes form a necklace on the breast, which is bolder on males than on females. Nearly 80% of the global population breeds in Canada, giving the country a large jurisdictional responsibility for conservation of this species.

Distribution

Canada Warbler breeds in all provinces and territories except Nunavut and Newfoundland and Labrador. In the United States, it breeds in several northern states and southward along the Appalachian Mountains. It migrates to overwinter in northwestern South America.

Habitat

Wet, mixed deciduous-coniferous forests with a well-developed shrub layer tend to be preferred for breeding, but Canada Warbler also uses riparian shrub forest on slopes and in ravines, and in stands regenerating after natural and anthropogenic disturbances. In its wintering range, Canada Warbler favours mature forest at altitudes of 1000 to 2000 m, but also occurs in second-growth forests, forest edges, shade coffee plantations, and other semi-open areas. During migration, the species most frequently occurs in woodlands with dense understory, including floodplain forests.

Biology

Canada Warbler breeds once annually, typically laying four to five eggs. Incubation lasts about 12 days. Chicks remain in the nest for 10 days, and are dependent on parents for 2 to 3 weeks after they leave the nest. Generation time is estimated to be 2 years.

Population sizes and trends

The Canadian population of Canada Warbler is estimated to be between 2 million and 10.4 million individuals. Breeding Bird Survey results for Canada indicate a cumulative decline of 51% between 1970 and 2019, but an improving trend since around 2003, including a cumulative increase of 46% between 2009 and 2019, which has restored the population to mid-1990s levels. Trends vary regionally in Canada, with the strongest short-term increases in the core of the range in the central and eastern boreal forest, contrasting with ongoing declines in Alberta and the Maritimes. Other data sources agree with a long-term decrease in Canada Warbler population size; the majority also indicate ongoing declines in the short-term, but in many cases the most recent data are at least 5-10 years old, largely preceding the recent increase documented by the Breeding Bird Survey.

Threats and limiting factors

Habitat loss and degradation on the South American wintering range in the Andes are thought to be the most likely factors responsible for the long-term decline of Canada Warbler. Within Canada Warbler’s wintering range, over 90% of the primary mountain forests have been cleared since the 1970s. In recent years there has been some regrowth at higher elevations in parts of the wintering range, but ongoing losses elsewhere and at lower elevations. Habitat loss has also occurred within parts of its Canadian breeding range, and along migration corridors, especially where wet forests have been drained for urban development or conversion to agricultural land (particularly in the east), or industrial expansion and road development (particularly in the west). During migration, evidence suggests the species is disproportionately vulnerable to collisions with tall buildings and other structures.

The need for forested wintering habitat in the northern Andes is a key limiting factor for Canada Warbler. The species shows some flexibility with regard to its ability to occupy shade-grown coffee plantations, but these are increasingly being converted to sun coffee cultivation, which is unsuitable for Canada Warbler.

Protection, status and ranks

Canada Warbler adults, nests and eggs are protected in Canada under the Migratory Birds Convention Act, 1994. It is also currently protected under Canada’s Species at Risk Act as a Threatened species. It is listed as Endangered under the Nova Scotia Endangered Species Act, Threatened under Manitoba’s Endangered Species and Ecosystems Act and the New Brunswick Endangered Species Act, and Special Concern under Ontario’s Endangered Species Act. It is also considered Vulnerable in British Columbia and is on the list of species likely to be designated threatened or vulnerable in Quebec. Canada Warbler is considered a species of high conservation concern by Partners in Flight in Canada and the United States. NatureServe ranks it as globally secure and the IUCN ranks it as Least Concern.

Technical summary

Cardellina canadensis

Canada Warbler

Paruline du Canada

Range of occurrence in Canada: British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, Yukon, Northwest Territories

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

Was a threats calculator completed for this species? Yes; overall threat impact high (see Appendix 1)

Key threats were identified as:

1. livestock farming and ranching (IUCN 2.3), primarily on the wintering grounds – medium threat impact
2. logging and wood harvesting (IUCN 5.3), primarily on the wintering grounds, but also to a lesser extent on the breeding grounds – medium threat impact
3. climate change and severe weather (IUCN 11), especially drought on the wintering grounds – low to medium threat impact
4. residential and commercial development (IUCN 1), notably collisions with tall – low threat impact
5. annual and perennial non-timber crops (IUCN 2.1), primarily on the wintering grounds – low threat impact
6. energy production and mining (IUCN 3) – low threat impact
7. utility and service lines (IUCN 4.2), especially collisions with communication towers – low threat impact
8. other ecosystem modifications (IUCN 7.3) – low threat impact

What other limiting factors are relevant?

Requirement for forest on its montane wintering grounds in the Andes

Data sensitivity

Is this a data sensitive species? No

Status history

COSEWIC Status History: Designated Threatened in April 2008. Status re-examined and designated Special Concern in November 2020.

Status and reasons for designation

Status: Special Concern

Alpha-numeric codes: Not applicable

Reasons for designation : This small songbird has 80% of its breeding range in Canada and winters in the northern Andes Mountains. Breeding Bird Survey results show that the long-term decline of the Canadian population began to slow down in 2003 and that numbers have increased steadily since 2012, with an overall growth of 46% over the past decade. However, significant threats persist, most notably clearing of forests in South America for livestock farming and other agriculture. The revised status reflects the substantial improvement in population trend since the previous assessment of Threatened, but concern remains that the species is at risk of becoming Threatened again if threats are not managed effectively.

Applicability of criteria

Criterion A (Decline in Total Number of Mature Individuals): Not applicable. Population has increased 46% over the past ten years.

Criterion B (Small Distribution Range and Decline or Fluctuation): Not applicable. EOO of 2,200,000 km² and IAO of >20,000 km² both exceed thresholds.

Criterion C (Small and Declining Number of Mature Individuals): Not applicable. Number of mature individuals is 2 to 10.4 million, exceeding thresholds.

Criterion D (Very Small or Restricted Population): Not applicable. Estimate of 2 to 10.4 million mature individuals exceeds thresholds for D1, and population is not vulnerable to rapid and substantial decline.

Criterion E (Quantitative Analysis): Not applicable. Analysis not conducted.

Preface

At the time of the first status report on Canada Warbler (COSEWIC 2008), the species was considered to be in the genus Wilsonia; it has since been reclassified as Cardellina.

Many studies have been undertaken on Canada Warbler since the previous status report. These have included research aimed at understanding migration routes, identifying important wintering areas, and establishing linkages to breeding areas (e.g., Cárdenas-Ortiz et al. 2017; González-Prieto 2018a,b; Roberto-Charron 2018a). Other studies have focused on better understanding habitat needs on both the breeding grounds (e.g., Haché et al. 2014; Ball et al. 2016; Grinde and Niemi 2016; Hunt et al. 2017; Westwood et al.2019) and the wintering grounds (e.g., González-Prieto et al. 2017; Céspedes and Bayly 2018; González-Prieto 2018a,b), as well as the role of landscape-scale disturbance during the breeding and non-breeding periods on demographic and population trends (Wilson et al. 2018). Collectively, the research undertaken over the past decade provides a better understanding of Canada Warbler ecology, and the relative importance of potential threats. Population trend estimates have also been refined and updated, including reanalysis indicating that the rate of decline at the time of the previous assessment was likely not as steep as reported then. Additional trend estimates have become available, including results from the Canadian Migration Monitoring Network and some second-generation breeding bird atlases.

Development of hemisphere-wide conservation efforts was launched in 2013 through creation of the Canada Warbler International Conservation Initiative (CWICI; Rosenberg et al. 2016). A national recovery strategy has been prepared (Environment Canada 2016).

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 (2020)

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)*

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)**

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

Data Deficient (DD)***

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 speces’ risk of extinction.

* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.

** Formerly described as “Not In Any Category”, or “No Designation Required.”

*** 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.

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

Species information

Name and classification

Scientific name: Cardellina canadensis
English name: Canada Warbler
French name: Paruline du Canada
Spanish name: Reinita Canadiense
Classification: Class: Aves, Order: Passeriformes, Family: Parulidae

This species was previously placed in the genus Wilsonia, but phylogenetic analyses showed that it is most closely related to other species in the genus Cardellina (Chesser et al.2011).

Morphological description

Canada Warbler is a small, colourful songbird. Males are more brightly coloured than females and immatures, with bluish-grey upperparts and tail contrasting with a yellow neck and throat. Males have a black forehead and cheeks, which join with a necklace of well-defined black stripes that run across the breast. Both sexes have prominent yellowish-white “spectacles” around the eyes. In females, the upperparts and tail are dull bluish-grey and the throat and breast are yellow with a somewhat lighter and less distinct necklace (Reitsma et al. 2009).

Population spatial structure and variability

No subspecies of Canada Warbler have been recognized (Reitsma et al. 2009), but preliminary results indicate some genetic differences between southern U.S.A. and eastern/western populations in Canada (Haché pers. comm. 2019; see below).

Designatable units

There is no geographic variation in morphology (Reitsma et al. 2009). Ferrari et al.(2018) found evidence that there might be subpopulations based on subtle genetic structuring, and preliminary unpublished results from another analysis suggested genetic differences between individuals from the United States, eastern Canada, and western Canada (Bossu and Ruegg 2019), perhaps reflecting genetic diversity that covaries with climatic variables. Isolation by distance seems to be the predominant mechanism of differentiation across the Canada Warbler’s breeding range. Four unique ecotypes of Canada Warbler may occur, defined as North Carolina, northeast (Maritimes), Quebec (central Canada), and western boreal. However, as yet there are no known distinctions that would warrant consideration of designatable units according to COSEWIC’s definition; therefore this report is based on the species as a whole (see also Dispersal and Migration).

Special significance

Canada supports nearly 80% of the global breeding population of Canada Warbler, giving it a large jurisdictional responsibility for conservation of this species. Aboriginal Traditional Knowledge was not available. However, Canada Warbler is part of an ecosystem that is important to Indigenous people who recognize the interconnectedness of all species.

Distribution

Global range

Canada Warbler breeds across the western and southern parts of the boreal ecoregion and much of southeastern Canada, extending south into the United States around the Great Lakes, northeastern states, and along the Appalachian Mountains as far south as Tennessee and Georgia (Reitsma et al. 2009; Figure 1).

Its winter range includes most of Venezuela, and the east slope of the Andes as far south as Peru (Robinson et al. 1995; American Ornithologists’ Union 1998; Figure 1). There are also some winter records from the foothills of Panama and Costa Rica. The greatest winter abundance is in the Andes of eastern Colombia, western Venezuela, northern Peru, and southern Ecuador (Reitsma et al. 2009; Roberto-Charron 2018a).

During spring and fall migration, records from the Caribbean islands are rather sparse, as the species seems to mostly follow an overland route through eastern Mexico and Central America (Cárdenas-Ortiz et al. 2017; Roberto-Charron 2018a; see Figure 1). Recent results from unpublished geolocator data suggest that a short-distance, partial trans-gulf migration (i.e., from coastal Texas to coastal Mexico) also occurs fairly commonly (Roberto-Charron pers. comm. 2019).

Canadian range

The Canadian breeding range of Canada Warbler extends from British Columbia to the Maritimes (Figure 1). Its range includes extreme southeastern Yukon (Sinclair et al. 2003; Bennett pers. comm. 2019), northeastern British Columbia (Phinney 2015), the extreme southwestern corner of the Northwest Territories (Machtans and Latour 2003), northern and central Alberta (Federation of Alberta Naturalists 2007), north-central Saskatchewan (Smith 1996), south-central Manitoba (Roberto-Charron 2018b), most of Ontario except the far south and far north (Cadman et al. 2007), south-central and southern Quebec (Robert et al.2019), New Brunswick, Prince Edward Island, and Nova Scotia (Stewart et al. 2015). It does not regularly occur in Nunavut or Newfoundland and Labrador.

Extent of occurrence and area of occupancy

Current extent of occurrence (EOO) is approximately 2.2 million km², calculated as a minimum convex polygon around the known breeding distribution per Figure 1. This is unchanged from the previous status report (COSEWIC 2008).

The index of area of occupancy (IAO) based on 2x2 km squares has not been calculated, as the specific distribution of the species is not known. The previous status report estimated a biological area of occupancy of 27,000 km² based on a population estimate of 1.35 million pairs and a mean home range of 2 ha (Chace 2005). By the same approach, the current equivalent would be 20,000 km² to 104,000 km².

Search effort

Recent data on the Canadian breeding distribution of Canada Warbler come primarily from breeding bird atlas projects carried out in British Columbia (Phinney 2015), Alberta (Federation of Alberta Naturalists 2007), Saskatchewan (Smith 1996), Manitoba (Roberto-Charron 2018b), Ontario (Cadman et al., 2007), Quebec (Robert et al. 2019), and the Maritimes (Stewart et al. 2015). Bird checklists submitted to eBird (Fink et al. 2018) also provide valuable data on relative abundance and distribution, including during the non-breeding period.

Habitat

Habitat requirements

Breeding habitat

Canada Warbler is found in a wide variety of forest types, with preferences varying considerably by region (Crosby et al.2019). In general, it is most common in wet, mixed deciduous-coniferous forest types having a well-developed shrub layer, often as a result of canopy gaps and suitable drainage and soil moisture conditions (Reitsma et al. 2009; Grinde and Niemi 2016; Churchill et al. 2017; Hunt et al. 2017). Across its range, Canada Warbler occurs variably in wet Red Maple (Acer rubrum) stands, cedar and spruce swamps, aspen stands, mixed aspen / birch / fir forests, beaver ponds, brushy slopes, riparian woodlands, and densely forested ravines (Peck and James 1987; Larue et al. 1995; Cooper et al. 1997; Drapeau et al. 2000; Chace 2005; Semenchuk 2007; Hallworth et al. 2008a; Reitsma et al. 2008; Chace et al. 2009; Stewart et al. 2015; Boone 2016; Westwood 2016; Roberto-Charron 2018b; Toussaint 2019; Westwood et al. 2019; Dufour-Pelletier et al.in prep.). Beaked Hazelnut (Corylus cornuta) and Mountain Maple (Acer spicatum) are particularly prominent in the shrub layer at sites favoured by Canada Warbler in central and eastern Canada (Dufour-Pelletier et al.in prep.). Hunt et al. (2017) reported that Canada Warbler occupancy is also heavily influenced by conspecific attraction.

Especially in western Canada, the species has an affinity for mature to old (>80 year) forest (Haché et al. 2014; Hunt et al. 2015; Phinney 2015; Ball et al. 2016; Grinde and Niemi 2016; Roberto-Charron 2018b); in Alberta, the relative abundance of Canada Warbler peaks in old (>140 year) deciduous forests (ABMI and BAM 2019). This can include upland forests, provided that there are canopy gaps that promote or allow for growth of a dense, well-developed shrub layer (Schieck et al. 1995; Enns and Siddle 1996; Cooper et al. 1997; Hobson and Bayne 2000a; Hobson et al. 2000; Schieck and Hobson 2000; Schieck et al. 2000; Cumming and Machtans 2001; Machtans and Latour 2003; Hannon et al. 2004, Krikun et al.2018).

In western Canada, Canada Warbler is typically most abundant in unharvested forest, but it can also be common in regenerating forests (i.e., 6–30 years post-disturbance) following fire, insect, or harvesting disturbances (Titterington et al. 1979; Welsh and Fillman 1980; Christian et al. 1996; Hobson and Schieck 1999; Drapeau et al. 2000; Hobson and Bayne 2000b; Schieck and Hobson 2000; Hunt et al.2015, 2017). In studies in northern Alberta, there was no evidence of reduced reproductive success in managed forest compared to protected areas, but population density was lower in harvested areas, and proximity to unharvested stands and conspecifics were important factors in determining the likelihood of use of harvested areas (Hunt et al. 2015, 2017).

Migration habitat

During migration, Canada Warbler favours areas with a well-developed shrub layer, such as forest edges, riparian habitats, and second-growth forests (Reitsma et al. 2009), seemingly with an affinity for floodplain forest (Power 1971), and typically foraging close to the ground in herbaceous and low woody vegetation (Keast 1980). In Mexico and northern Central America, it uses the shrub layer and upper layers of humid to semi-humid forests and forest edges from sea level to 2500 m (Binford 1989; Howell and Webb 1995). In Honduras and Panama, it occurs in open forests, second-growth forests, shrubland habitat, and mangrove forests (Monroe 1968).

Winter habitat

In Colombia, Canada Warbler occurs in mountainous areas and foothills at 750–2300 m, being most abundant at elevations between 1000-2000 m and occurring at higher densities in mature forest than in secondary forest, shade coffee plantations, forest edges, or other semi-open areas (Céspedes and Bayly 2018). In forest habitat, birds foraged at mid-levels in the canopy (5-15 m above ground). In one Colombian study, Canada Warbler was not found in landscapes having less than 20% forest cover (Colorado 2011).

In Peru and Ecuador, Canada Warbler uses rainforests on the east slope of the Andes and the adjacent lowlands (Paynter 1995). In the Venezuelan Andes, it is found in humid to wet montane forests and older second-growth forests (Hilty 2003). Throughout, it is fairly common in shade coffee plantations (Bakermans et al. 2009; González-Prieto 2018a), though these appear to be of lesser quality than natural forests (see Physiology and Adaptability).

Habitat trends

Breeding range

Over the past century or more, some of the forested wetland habitat in the species’ breeding range from southern Ontario east to the Maritimes has been drained (Tiner 1984; Reitsma et al. 2009) or converted to agriculture or urban areas (Gauthier and Aubry 1996). However, more recent regeneration of forest in this region following clearing in the early to mid-1900s has reclaimed patches of habitat for Canada Warbler (Reitsma et al. 2009). Habitat loss within the species’ range in the boreal ecoregion has been more localized and related primarily to agricultural expansion, road construction and, especially in the western part of its range, also oil and gas drilling, oil sands development, and other industrial infrastructure (SSBF 1999; Hobson et al. 2002; Schneider et al. 2003; AESRD and ACA 2014; Westwood et al. 2019). In some areas, over-browsing by ungulates is also reducing the shrub layer, potentially reducing the suitability of some forests for Canada Warbler (Cardinal et al.2012).

Net loss of forest in Canada has been ongoing, but the rate has dropped from about 64,000 ha/year in 1990 to about 37,000 ha/year in 2016, with less than 0.5% of Canada’s total forest area converted to other land uses over this period (Natural Resources Canada 2018). The conversion of old growth forest and old even-growth forest to younger stands may be of greater concern, especially in the western part of the breeding range (Hobson and Bayne 2000a). Near the end of the 20^th century, 60-70% of harvesting in northwestern Canada was occurring in old-growth stands (SSBF 1999).

While long-term (permanent) loss of forest cover from logging activities is largely offset by natural regeneration, tree planting, and other forest management activities (CCFM 2019), pre- and post-harvest management practices that reduce the shrub layer could adversely affect the quality of Canada Warbler habitat in managed areas (Gauthier and Aubry 1996). Further studies are needed to assess the impact of silvicultural practices on the suitability of managed forests for Canada Warbler.

Winter range

The forests of the northern Andes (primarily in Colombia), between 500 and 2000 m in altitude, were considered by Davis et al.(1997) to be among the most threatened forests in the world. By the early 1990s, about 90% of all primary forest in the northern Andes, including 95% of the cloud forest, had been cleared for agriculture, fuel wood, cultivation of illegal drugs, and herbicide spraying to eliminate these drug crops (Henderson et al. 1991; Davis et al. 1997). In Colombia alone, the rate of deforestation in the early 2000s was about 2000 km² per year (Butler 2006), with only 9.6% of the 2,763,523 ha of original forest in the Colombian Andes within the 1500–2000 m elevation range remaining intact (Rodriguez et al.2004). Expansion of cattle pastures since the mid-1800s has been a major driver of habitat change in the Andes (Dillon 1994; Etter et al. 2008).

Recent habitat trends have been more mixed. There was some expansion of forest cover in Colombia over the period 2001 to 2010 (Sánchez-Cuervo et al. 2012). Aide et al.(2019) noted that from 2001 to 2014, forest loss in the Andes was most prevalent between 1000 and 1499 m elevation, whereas forest gains were observed above 1500 m. Given that Canada Warbler winters at elevations of 750 to 2300 m, and is most abundant between 1000 and 2000 m (Céspedes and Bayly 2018), the overall implications of these changes are somewhat unclear. Recent satellite monitoring observations by the World Resources Institute show that over 421,000 ha of tree cover were lost in Colombia in 2017, a 46% increase over 2016 (Clynes 2018). The recent declines are believed to be related in part to a rise in land clearing following the 2016 signing of the peace accord between the Colombian government and the Revolutionary Armed Forces of Colombia (FARC) (Salazar et al. 2018; Prem et al. 2020). Prior to signing of the accord, security risks limited economic activity in many regions of Colombia, including Andean regions that provide wintering habitat to Canada Warbler. Increased security since implementation of the accord has stimulated economic activity, particularly from larger industrial operations (Prem et al. 2020), resulting in loss of forest cover; these trends are projected to increase (Salazar et al. 2018). The Andean regions of Colombia are also one of the primary regions in Latin America where habitat for Neotropical migrant songbirds (e.g., mature and secondary forest, mosaic forest-cropland) is expected to be lost to open agriculture between now and 2050 (Wilson et al. 2019). One such cause is that traditional shade coffee plantations, which Canada Warbler uses frequently, are increasingly being replaced by coffee grown in full sun, which does not provide habitat for this species (Perfecto et al. 1996; Escobar 2013; González-Prieto 2018a,b; Wilson et al.2019).

Migration habitat

In the southern U.S. and Mexico, forest loss along the Gulf coast has been occurring due to urban and commercial development, agriculture, livestock grazing, logging, and the spread of exotic species (see Barrow et al.2005).

Biology

Life cycle and reproduction

Generation time for Canada Warbler is estimated at 2 years by BirdLife International (Bird et al.2020), taking into account its age at first breeding (1 year; Reitsma et al. 2009), maximum life span (~8 years; Klimkiewicz et al. 1983), and average annual adult survival rate of 0.51, considered low for a small landbird (Albert et al. 2016). This estimate of adult survival is consistent with the estimate of 0.47 (std = 0.027; CV = 5.7%; range = 0.44 to 0.54) derived from the Monitoring Avian Productivity and Survivorship program (MAPS; DeSante et al. 2015). Wilson et al.(2018) found apparent survival probability to be higher in the central part of the breeding range, compared to western or eastern.

Nests are on or near the ground and used only once. They are well-concealed by dense, low vegetation (e.g., moss hummocks, upturned tree roots, small hillocks with deep leaf litter and dense saplings) and are difficult to find (Reitsma et al. 2008, 2009; Goodnow and Reitsma 2011).

In high-quality breeding habitat, Canada Warbler breeding territories are often clumped near one another in “neighbourhoods” (Reitsma et al. 2009; Hunt et al. 2017). In various studies, average territory sizes range from 0.2 ha to about 1 ha (Chace 2005; Machtans 2006; Hallworth et al. 2008a; Reitsma et al. 2009; Flockhart et al. 2016). Average home range size ranges from 1 to 2 ha (Chace 2005; Hunt et al. 2015).

The egg and nestling stages in Canada can last from the end of May to end of July (Rousseu and Drolet 2017). The typical clutch size is four to five eggs, with one brood produced annually (Peck and James 1987; Reitsma et al. 2009). Incubation lasts about 12 days; chicks remain in the nest for about 7-9 days, and are dependent on parents for 2 to 3 weeks after they leave the nest (Reitsma et al. 2009). Fledging brood size averages about 3.8 young (Reitsma et al. 2009).

Physiology and adaptability

Canada Warbler requires a well-developed shrub layer in its breeding habitat. Given this, it exhibits a certain degree of adaptability to human disturbances such as forest harvesting, as well as natural disturbances (e.g., fire, wind, insect defoliation).

 Harvest intensity (e.g., single-tree selection versus clearcuts) is important. Canada Warbler readily occupies forests that are regenerating following harvesting, particularly stands between 6-20 years post-harvest in the east (Lambert and Faccio 2005) and 20-30 years post-harvest in the west (Hobson and Schieck 1999; Schieck and Hobson 2000; Hunt et al. 2015). This is because the shrub layer peaks during early regeneration stages (Sodhi and Paszkowski 1995; Norton and Hannon 1997; Tittler et al. 2001). Similarly, it occupies stands recovering from fire (Hobson and Schieck 1999; Schieck and Hobson 2000), although it tends to be less abundant there than in stands regenerating from harvest (Hobson and Schieck 1999).

Canada Warbler appears to be relatively tolerant of habitat fragmentation that results from forest harvesting (Schmiegelow et al. 1997; Ball et al. 2016). This may be because a well-developed shrub layer often regenerates adjacent to the fragments following harvesting (Schmiegelow et al. 1997), but occupancy is influenced by the availability of surrounding suitable habitat (Sólymos et al.2015). Canada Warbler is less tolerant of forest fragmentation associated with agriculture, which has a more permanent impact (Robbins et al. 1989; Hobson and Bayne 2000c).

On its wintering grounds, Canada Warbler makes heavy use of shade coffee plantations (Finch and Stangel 1993; Bakermans et al. 2009; González-Prieto 2018a,b). Compared to natural forest, coffee plantations appear to offer less food over the course of the winter (González-Prieto 2018a); however, no differences in apparent annual survival were observed between individuals that overwintered in shade coffee versus native forests (González-Prieto et al. 2020).

Canada Warbler feeds primarily on flying insects (e.g., Diptera and Lepidoptera) and spiders in the shrub layer, which explains a previous common name – “Canadian Flycatcher” (Reitsma et al. 2009). It uses a variety of foraging techniques, including fly-catching, sallying, hover gleaning, foliage gleaning, and ground gleaning (Reitsma et al. 2009). Although not considered a Spruce Budworm (Choristoneura fumiferana) specialist, Canada Warbler has been reported to respond to budworm outbreaks (Crawford and Jennings 1989; Sleep et al. 2009). These studies were correlative and later refuted by Venier and Holmes (2010) and Venier et al. (2012). A more recent study concluded that there may be local or regional effects on Canada Warbler populations resulting from budworm outbreaks, but that they are perhaps driven by short-term changes in forest structure, such as proliferation of shrub growth following tree defoliation (Drever et al. 2018). Walker and Taylor (2020) also found a positive correlation between Spruce Budworm indices and both spring and fall trends.

Dispersal and migration

Territory fidelity was considered high by Reitsma et al. (2009). Average site fidelity of adult males returning in one study in New Hampshire was estimated to be 52% across four years (Hallworth et al. 2008a). Because this was a single study area, these results for fidelity do not necessarily contradict the lower survival results (47%) reported earlier from a much larger geographic scale and time period (see Life Cycle and Reproduction above).

Canada Warbler is a long-distance (neotropical), nocturnal migrant (Reitsma et al. 2009). It arrives on its breeding grounds later in spring than many other species of wood warbler, and departs on its southbound migration early in fall (Flockhart 2007; McLaren 2007). Spring arrival is typically between mid-May and early June in most of Canada, while fall migration can begin as early as mid-July, and overall peaks in Canada from late August to early September (NatureCounts 2020; Bégin-Marchand in review).

The species may exhibit some migratory connectivity between its breeding and wintering grounds, but the pattern is not clear. Based on results from feather isotope analyses, Canada Warblers originating from the eastern part of the species’ breeding range tend to overwinter in the eastern Andes, whereas birds wintering in the western Andes tend to breed in western North America (González-Prieto et al. 2017; González-Prieto 2018a). This geographic segregation, coupled with differential rates of loss/degradation of wintering habitat in the eastern and western Andes, could explain the stronger population declines observed in eastern North America than in the west (Wilson et al. 2018; see Fluctuations and Trends). However, results from a tracking study using light-sensitive geolocators found that individuals from western, central, and eastern parts of the breeding range all funnelled through a small area along the western Gulf of Mexico, with 72% (N = 25) of them overwintering in Colombia, and no evidence of population-specific migration routes (Roberto-Charron 2018a, 2020). Both technologies have limitations and imprecision, so more research is needed to better determine the strength of migratory connectivity of the species.

Interspecific interactions

No specific information is available on interactions with predators (Reitsma et al. 2009), though Red Squirrel (Tamiasciurus hudsonicus) is thought to be an important nest predator in Alberta (AESRD and ACA 2014; Flockhart et al. 2016).

Peck and James (1987) reported a 20% rate (N = 25) of parasitism of Canada Warbler nests by Brown-headed Cowbird (Molothrus ater). However, cowbird parasitism is not thought to be a major factor affecting nest success of boreal songbirds (AESRD and ACA 2014), and the overlap in peak abundance between the two species is relatively minor (Cadman et al.2007). Although Canada Warbler is regularly parasitized in more southerly regions (see Reitsma et al. 2009), the extent to which this is a factor in Canada is likely more limited.

Like other forest songbirds, Canada Warbler is territorial during both the breeding and wintering seasons. Antagonistic interactions occur with other warblers during the breeding season, and within mixed-species flocks during dispersal, migration, and on the wintering grounds (Ridgely and Gwynne 1989; Hobson and Van Wilgenburg 2006; Reitsma et al. 2009), but have not been reported to have any population level impacts.

Population sizes and trends

Sampling effort and methods

Several bird monitoring programs capture information on abundance and/or distribution of Canada Warbler and can be used to estimate population trends. In addition to the sources listed below, eBird is anticipated to soon become a valuable tool for such analyses (e.g., Walker and Taylor 2017; Horns et al.2018), but results are not yet available for Canada Warbler.

The Breeding Bird Survey (BBS)

The Breeding Bird Survey (BBS) is a large-scale program that monitors breeding bird populations across North America (Hudson et al. 2017). Breeding bird abundance data are collected by volunteers at 50, 400-m radius stops spaced 0.8 km apart along permanent 39.2-km routes. In Canada, the surveys usually take place in June during the breeding season of most forest birds and are conducted from 0.5 hour before to about 4.5 hours after sunrise. The BBS provides medium reliability in terms of geographic coverage for Canada Warbler in Canada (Smith pers. comm. 2018), and there are issues with bias associated with roadside detection for this species (Haché et al. 2014). Still, it is the best available source of population trend information.

Breeding bird atlas projects

Provincial and state breeding bird atlas projects are normally carried out within a period of about five years and are designed to be repeated at 20-year intervals. They provide 20-year snapshots of species’ breeding distribution and abundance. When repeated, they also give information on change in both the extent and area of occupancy. In Canada, atlas projects have been completed (or have begun) in all provinces except Newfoundland and Labrador. To estimate change in status, atlas projects must be repeated; such repeat surveys have occurred only in Alberta, the Maritimes, Quebec, and Ontario, in addition to several states in the U.S.

The second Ontario breeding bird atlas compared the province’s distribution of breeding birds between 1981-1985 and 2001-2005 (Cadman et al. 2007). Data were gathered by volunteers who visited 10 x 10-km squares during the breeding season. The percent change in rate of occurrence in Ontario over the period of 20 years was then calculated by comparing the percentage of the 10 x 10-km squares with breeding evidence in the first atlas period to the percentage of squares with breeding evidence in the second atlas, adjusting for observation effort (Cadman et al. 2007). This program covered the entire breeding range of the species in Ontario.

Second breeding bird atlas projects that have been carried out in the Maritimes (2006-2010; Stewart et al. 2015) and Quebec (2010-2014; Robert et al.2019) are based on similar methodologies to those developed in Ontario. Because atlas projects provide snapshots across long time periods (usually at 20-year intervals), they are not sensitive measures of short-term change.

Ontario Forest Bird Monitoring Program (FBMP)

The Ontario Forest Bird Monitoring Program began in 1987 and is coordinated by the Canadian Wildlife Service. Its objective is to document forest bird population trends and the relationships between the birds and their habitat during the breeding period in relatively unfragmented forest landscapes (Canadian Wildlife Service 2008). Volunteers use point counts to survey birds in both large forest areas and forest fragments. One of the limitations of this program for monitoring Canada Warbler is that the species occurs at relatively few sites. In addition, the survey is biased towards relatively intact, unmanaged mature forests. The most recent trend estimates currently available are from 2011 (Cadman pers. comm. 2019).

Canadian migration monitoring network

This program is designed to monitor populations of migratory passerines at a series of over 20 monitoring stations across Canada. Fall migration, in particular, reflects the annual productivity and abundance of birds breeding in the boreal forest, as they move from their northern breeding grounds to their southern wintering areas (Crewe et al. 2008). The primary activities carried out at these stations are daily bird banding and visual counts of birds during spring and/or fall migration. The program assumes that the number of birds detected at each station is proportional to the actual number of birds migrating on the days monitored. A major limitation is that the geographic origin of birds is not well understood, especially for northern species like Canada Warbler. Only stations and seasons with adequate data to monitor Canada Warbler migration were considered, i.e., those covering at least 75% of the migration window, and with a clear migratory surge during the station’s coverage period (3-4 times the level of local birds pre- and/or post-migration).

Boreal Avian Modelling (BAM) project

The Boreal Avian Modelling (BAM) Project was developed to improve understanding of the ecology of boreal birds and their habitats, and to project the impacts of climate change and industrial development on bird populations and distribution (Boreal Avian Modelling Project 2019). It is built on a vast data set derived from tens of thousands of bird point-counts that have been collated from a large number of partners from governments, environmental organizations, industries, consultants, and academia, including the BBS and breeding bird atlases. BAM is most useful for studying patterns of relative abundance across the boreal forest and for investigating habitat relationships, but may not be as suitable for population trend analysis because data are less uniformly standardized than the BBS and are primarily from a shorter time period. As with the BBS, a potential source of bias is that most point counts in the database are conducted on or near roadsides, although BAM analyses allow for this factor to be accounted for in models.

Abundance

According to the most recent abundance estimates derived largely from BBS data, Canada Warbler populations in North America total 2.6 million individuals, with about 2 million occurring in Canada, or about 77% of the global population (PIF 2019). The most recent analysis by BAM estimates a range of approximately 9.2 to 10.4 million individuals in Canada (BAM 2020). This difference may be a function of the greater roadside bias of the BBS, coupled with differences in how detection distances were estimated (Haché et al. 2014). Which estimate is most accurate is uncertain, but all results greatly exceed COSEWIC quantitative thresholds for status assessment based on population size. It is important to note that the sources of bias underlying these differences have a greater influence on estimates of absolute abundance than they do on trends; therefore the discrepancies in population size do not undermine the reliability of population trends derived from the BBS.

The latest Partners in Flight abundance estimates show that breeding populations are largest in Ontario, Quebec, and Manitoba, which together comprise 79% of the Canadian total (see Table 1; PIF 2019).

Fluctuations and trends

Breeding Bird Survey

Although there is a roadside bias to the BBS, it is nonetheless considered the best available estimator of Canada Warbler population trends as it provides the greatest spatial and temporal coverage of all surveys, and any biases are expected to have remained relatively consistent over time. Long-term BBS data for Canada show an average annual trend estimate of -1.46%/year (95% Credible Interval [CI] = -2.29%, -0.61%) between 1970 and 2019, amounting to a cumulative long-term change estimate of -51.4% (95% CI = -67.8, -26.1; Table 2). At a provincial scale, long-term declines are steepest in the Maritimes and Quebec (Table 2).

Over the last decade (2009-2019), BBS data indicate that declines have reversed in some parts of Canada including Quebec, northern and southern Ontario, northern and eastern Manitoba, and Saskatchewan, but that declines continue in Alberta, central Ontario, and the Maritimes (Figure 2). BBS data for the most recent 10-year period (2009-2019) estimate an average annual increase of 3.85%/year (95% CI = 1.14% to 7.17%; Table 2), which amounts to an increase of 45.9% (95% CI = 12.0, 99.8) over the 10-year period, with zero probability of a reduction of >30%. The annual rate of increase was highest in Quebec, at 7.39% (95% CI = 2.81, 12.91), corresponding to a cumulative increase of 104% (95% CI = 32, 237). Only New Brunswick has continued to experience a substantial decline (-8.02%/year, 95% CI = -12.34, -3.32) during this time span, corresponding to a 0.97 probability of a reduction of >30%. At a national scale, the plot of annual indices of abundance indicates that population size as of 2019 is comparable to levels from the mid-1990s (Figure 3).

Rolling 10-year population trends for Canada Warbler in Canada based on BBS data from 1995 to 2019 show they have been improving annually over the past 15 years, and have been consistently positive since 2012 (Figure 4). At the time of the previous status report, the short-term trend was reported as -43% over 10 years (1997-2007; COSEWIC 2008). However, revised analysis of BBS data using a newer generalized additive model approach, which provides more accurate estimates of short-term trends than the previous method, indicates that the value at that time was actually -23% (Figure 4).

Breeding bird atlas projects

Repeat breeding bird atlas projects conducted in eastern North America generally show a decline in Canada Warbler occupancy rates over approximately 20-year time periods (see Table 3). Converted to 10-year time frames, the Canadian values in Table 3 give losses of 5% to 11%. This is indicative of regional reductions in measures of both area of occupancy and in abundance. However, the Maritimes and Ontario results largely to entirely precede the 2009-2019 short-term assessment window, and correspond to a period when BBS data were also indicating declines. The second Quebec atlas is somewhat more recent, and the decline in probability of observation relative to the first Quebec atlas contrasts with the increase observed through the BBS. Direct comparison is complicated though, as the BBS trend appears to be driven by particularly high counts in the five years after the second atlas survey period ended.

^a Maritimes atlas results are raw changes in number of squares occupied, not a change in probability of occurrence.

Ontario Forest Bird Monitoring Program

Population trends calculated from this program showed a statistically significant decline of 6.3%/year (LCI = -9.4; UCI = -3.0; n = 545) for the 25-year period between 1987 and 2011 for Ontario (Cadman pers. comm. 2019). This is equivalent to a decline of about 79% for the entire time series. However, population trends for this monitoring program have not been calculated since 2011, so it is not possible to determine whether it would reflect the change to increasing trends observed in BBS data after 2011 (see above).

Canadian Migration Monitoring Network

Trend results from migration monitoring stations are variable. Long-term data are available only for Long Point Bird Observatory, where statistically non-significant trends of -0.7%/year in fall and 0.1%/year in spring were recorded over the period 1968 to 2016. For the 10-year period from 2006-2016, eight migration monitoring stations have trend information available. Trends are negative at sites in Saskatchewan and Ontario, which primarily sample individuals from the central to western parts of the Canadian breeding range; the lone positive trend is from McGill Bird Observatory in Montréal, which largely encounters those breeding in Quebec (Crysler pers. comm. 2019; Table 4).

Summary of trends

In summary, results from the Breeding Bird Survey, which provide the best trend information on Canada Warbler populations across Canada, show widespread and substantial long-term declines. At the time of the previous status report, the short-term decline based on BBS data was -43% over ten years, but revised analysis suggests the rate was actually -23% (Figure 4). The most recent short-term trend (2009-2019) indicates a considerable increase of 46%, although there is regional variability, with increases from Saskatchewan through Quebec, spanning the core of the Canadian range with approximately 85% of the population, but ongoing modest decreases in Alberta, Nova Scotia, and Prince Edward Island, and an accelerating decline in New Brunswick. Canadian Migration Monitoring Network data differ in showing slight to serious declines in the central and western part of the breeding range, with an increase only in Quebec. The particularly strong population growth in Quebec correlates with a large Spruce Budworm outbreak over the past decade (National Forestry Database 2018; see Figure 5), although it remains unclear to what extent Canada Warbler benefits from such episodes. The observed national population increase may also be related to some recent regrowth of high-elevation forest cover in Canada Warbler’s wintering range in South America (Aide et al. 2019).

Rescue effect

In the event of the extirpation of the Canadian population, immigration of individuals from a number of U.S. border states is possible. In the United States, Canada Warbler has increased at 1.73%/year (95% CI = -0.20% to 3.87%; N = 351 routes) over the most recent 10-year period from 2009 to 2019, although growth has been strong only in Minnesota and New York state, with stable to declining numbers elsewhere (Environment and Climate Change Canada, unpubl. data). Immigration would be possible and immigrants would be adapted to survive in Canada. Given that the population is currently increasing both in Canada and the U.S. it is difficult to predict the likelihood of rescue under a scenario of severe decline in the Canadian population, as such a change could also affect the outside population.

Threats and limiting factors

Canada Warbler is vulnerable to the cumulative effects of various threats throughout its annual cycle. These factors 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 impacts for 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), 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.

For Canada Warbler, the overall threat impact is considered to be high, corresponding to an anticipated decline of between 10 and 70% over the next 10 years (see Appendix 1 for details). However, the actual rate of change is expected to be closer to the low end of this range, given that the population has been steadily increasing despite the recognized threats largely being ongoing. Threats are discussed below, in order of decreasing severity of impact.

Threats

IUCN 2, Agriculture and aquaculture (medium threat impact) and IUCN 5, Biological resource use (medium threat impact):

IUCN 2.1, Annual and perennial non-timber crop (low threat impact),
IUCN 2.3, Livestock farming and ranching (medium threat impact), and
IUCN 5.3, Logging and wood harvesting (medium threat impact):

Description of threat

These threats are discussed jointly, as Canada Warbler winters in areas where deforestation is intensive, but it can be difficult to differentiate whether the underlying motivation is to expand agricultural crops, increase livestock ranching, or simply undertake forestry operations (Reitsma et al. 2009; Clynes 2018). The forests of the northern Andes (primarily in Colombia), which are the main wintering grounds of Canada Warbler, are among the most threatened in the world (Davis et al. 1997; Rodriguez et al. 2004), and habitat loss there has been identified as the greatest threat to the species (Lambert and Faccio 2005; Wilson et al. 2018). For example, over the period 1993-2009, Wilson et al. (2018) calculated that an index of the “human footprint” on the breeding grounds increased by only 0.1%, compared to 14% on the wintering grounds, although their analysis excluded effects of forestry. Over 90% of forest cover in the northern Andes has been lost (see Habitat Trends), and the Food and Agriculture Organization (2010) reported ongoing forest cover loss in the Andes of 50,000 to 500,000 ha per year. Natural forest provides the best wintering habitat for Canada Warbler. A secondary wintering habitat is shade-coffee plantations, but their extent is being reduced through conversion to ranching, or coffee grown in full sun (Perfecto et al. 1996; Escobar 2013; González-Prieto 2018a,b), which is not nearly as suitable for Canada Warbler.

In the breeding range, Canada Warbler exhibits a degree of adaptability to human disturbance. Cumulative effects of forest harvesting, fire, and energy sector development are negative, but can be counter-balanced by a positive response to shrub succession following selective forest management and natural disturbances (e.g., Hallworth et al. 2008a,b; Reitsma et al. 2009; Leston et al.2020). Silvicultural practices that adversely affect the development of the shrub layer in managed forests may decrease habitat for the species, but again mostly in the short term (Askins and Philbrick 1987; Cooper et al. 1997; Norton and Hannon 1997; Schieck et al. 2000; Tittler et al. 2001; Hunt et al. 2015, 2017). Although effects from forest management are not consistent across studies (Reitsma et al. 2009; Haché et al. 2014), the species generally appears to respond positively to selective logging (Hallworth et al. 2008b; Becker et al. 2012). Abundance may subsequently decline again after the regenerating shrubby understory grows above a certain height (e.g., Hagan and Meehan 2002; Grinde and Niemi 2016).

Draining of swamp forests for agriculture (and to some extent also for urbanization), especially between 1950 and 1980, is thought to have contributed to the historical decline of Canada Warbler populations in eastern North America, especially south of the boreal region (Tiner 1984; Miller 1999; Reitsma et al. 2009). Although still ongoing, the extent and rate of forest swamp drainage has decreased. Ongoing loss of mature forest (especially associated with riparian areas) remains a concern especially in the western boreal region (Ball et al. 2016). Boreal mixedwood forest has been cleared for agriculture in western Canada (SSBF 1999; Hobson et al. 2002), as well as industrial development (e.g., road and pipeline construction, drilling sites, oil sands development) in the oil and gas sector (Cooper et al. 1997; SSBF 1999; Hobson et al. 2002; AESRD and ACA 2014; Ball et al. 2016; see Habitat Trends).

Information on the effects of forest habitat fragmentation on Canada Warbler is mixed. Some studies suggest that the species is sensitive to fragmentation (Askins and Philbrick 1987; Robbins et al. 1989; Litwin and Smith 1992; Hobson and Bayne 2000c; Westwood 2016); others indicate that the species is tolerant of habitat fragmentation that results from forest harvesting (Schmiegelow et al. 1997; Schmiegelow and Monkkonen 2002), possibly because of the high rate of temporary forest/shrub regeneration that occurs within the regenerating stands.

Scope

Within the next decade, the proportion of the wintering range likely to be affected by some form of forest loss is anticipated to be in the range of 11-30%. In the breeding range, only 24% of the species’ breeding distribution in the boreal forest is in areas exposed to anthropogenic disturbance (Wells 2011), suggesting that the scope of exposure during the breeding season is likely also restricted at most.

Severity

Loss of forest cover in the wintering range for any reason is likely to have a serious effect, as there is limited alternate habitat available. Evidence from the breeding range suggests that forest loss there may have both positive and negative short-term effects, and severity there is overall unlikely to be more than slight.

IUCN 11, Climate change and severe weather (low to medium threat impact)

Description of threat

Price et al. (2013) predicted that boreal forests will be highly altered by climate change. Over the long term, changes in forest composition due to climate change could drive Canada Warbler’s breeding range northward (e.g., Matthews et al. 2004; Stralberg et al. 2015). The potential for increasingly severe winter storms resulting in wind throw or ice damage may create canopy gaps that facilitate shrub growth in forests (Faccio 2003). Whether any of these changes will result in a net loss of population size is unknown, because there is potential for a compensatory growth of population at the northern extreme of the species’ breeding range.

The impact of climate change may vary depending on the extent of forestry activities. Under the worst case scenario for climate change and high harvesting, Canada Warbler abundance in northeastern Alberta is predicted to decline by 17%, compared to an estimated 39% increase under the same conditions with no harvest (Cadieux et al.2020). Simulations for eastern Canada also suggest that choice of harvesting strategies is likely to have a greater influence on population trends than climate change (Bognounou et al. in prep).

Spring migration arrival dates advanced slightly but not significantly for Canada Warbler in an investigation of climate change effects in the Great Lakes Basin (Marra et al. 2005). A more recent study reported that Canada Warbler populations in the same general region of the Great Lakes were moderately vulnerable to climate change risks (Marra et al. 2014).

On the winter grounds, González-Prieto (2018a) and Gonzalez-Prieto et al. (2020) suggested that more frequent drought conditions induced by climate change could compound the consequences of drought arising from deforestation (Bagley et al.2014), including loss in quality of winter habitat, body condition, and potentially annual survival of Canada Warbler.

Scope

Nearly all of the Canada Warbler population will be exposed to habitat shifting and alteration and to increased risk of drought.

Severity

More research is required to understand the implications of habitat shifting and alteration, temperature extremes, and storms and flooding on Canada Warbler. Effects of drought also remain only partially understood, but are likely to range from slight to moderate.

IUCN 1, Residential and commercial development (low threat impact)

Description of threat

Canada Warbler has historically lost breeding and migratory stopover habitat to urban development, and to a minor extent this continues. However, for this species a greater risk associated with buildings is the risk of collisions during nocturnal migration, especially with tall illuminated structures such as office towers. Although not a particularly abundant species overall, it was among the top ten for collision mortalities in a large study across the U.S. (Loss et al. 2014), and communication towers were estimated to kill 1.5% of the North American population annually (Longcore et al.2013). Based on daily lighthouse kill data collected at the Long Point Bird Observatory (Ontario) in spring and fall from 1960 to 2001, it was also in the top 15 species killed there (J.D. McCracken, unpubl. data). With the proliferation of tall buildings and communications towers, this threat continues to increase in scope (see Calvert et al. 2013 for an overview of anthropogenic threats). Collisions with windows of residential buildings during migration also represents a threat.

Scope

The proportion of Canada Warblers encountering this threat is believed to be small, although it is difficult to quantify.

Severity

Of those that encounter collision risks with buildings, most are likely unaffected, but given the relative frequency of mortality reports, the overall severity is probably sufficient to be considered slight rather than negligible.

IUCN 3, Energy production and mining (low threat impact)

Description of threat

Oil and gas drilling and mining can affect Canada Warbler through removal of habitat, as well as displacement arising from sensory disturbance. Mortality through collision with wind turbines is possible, but not documented as a particular concern.

Scope

Oil and gas drilling (IUCN 3.1) occurs across parts of Canada Warbler’s western breeding range, but population density is lower there than in eastern Canada, and likely far fewer than 10% of individuals are exposed to this threat. Exposure to mining and quarrying (IUCN 3.2) is even more limited, and considered negligible. A small portion of the population likely migrates through renewable energy projects (IUCN 3.3).

Severity

Displacement by oil and gas drilling, mining, or quarrying would likely have a moderate effect at a local level, although modelling of effects on recruitment suggested a low impact (Van Wilgenburg et al.2013). The likelihood of collision with wind turbines is lower, and is considered slight, but may even be negligible.

IUCN 4, Transportation and service corridors (low threat impact)

Description of threat

Several studies have shown that breeding density of Canada Warbler is lower near roads (Miller 1999; Schneider et al. 2003; Haché et al. 2014; Westwood 2016; Westwood et al. 2019). Conversely, Becker et al. (2012) found a positive relationship with roads in Virginia, but their study might have been influenced by detection bias and/or a correlation between roads and canopy disturbance favouring the species (Roberto-Charron pers. comm. 2019). Regrowth of abandoned logging roads can provide suitable habitat for Canada Warbler, especially in southern boreal forests. Overall, the severity of roads appears to be negligible.

Collisions with communication towers and associated guy wires is likely a somewhat greater concern (see IUCN 1). This is particularly problematic where lights attract birds toward these hazards. The increasing demand for communication network upgrades means that the number of towers will continue to grow, and the threat will increase unless mitigation measures are improved.

Scope

A restricted portion of the population is likely exposed to utility and service lines; considering a tendency to avoid linear disturbances, only a small portion of the population may be affected by roads and railroads.

Severity

Mortalities from communication towers are sufficiently frequent to suggest a severity score of moderate. Roads and railroads likely have negligible effects.

IUCN 7, Natural system modifications (low threat impact)

Description of threat

Effects on Canada Warbler under this category are likely limited to other ecosystem modifications (IUCN 7.3), as the severity of fire and fire suppression (IUCN 7.1) is unknown, and the scope of dams and water management/use (IUCN 7.2) is negligible. Herbicide application and changes in drainage resulting from forestry, logging roads, and other landscape modifications may result in changes to vegetation structure that reduce suitability for Canada Warbler. In the southern part of its range, Canada Warbler may be affected by White-tailed Deer overbrowsing the shrub layer (DeGraaf et al.1991). The death of Eastern Hemlock (Tsuga canadensis) due to Hemlock Woolly Adelgid (Adelges tsugae) has been identified as a threat to Canada Warbler in Ohio through loss of habitat (Boone 2016). Hemlock Woolly Adelgid has reached parts of southern Ontario and Nova Scotia since 2013, and may become an increasing concern in Canada (Government of Canada 2019a).

Scope

Collectively, other ecosystem modifications likely affect only 11-30% of the Canadian population.

Severity

Other ecosystem modifications are expected to have a moderate effect, primarily through reduced productivity.

Limiting factors

The biggest limiting factor appears to be the concentration of Canada Warbler’s winter range in the northern Andes, and the restricted amount of remaining forest habitat there.

Number of locations

Number of locations for Canada Warbler is unknown, but considering that the greatest threats are related to forest loss on privately owned land, there are likely far more than 10.

Protection, status and ranks

Legal protection and status

In Canada, Canada Warbler individuals and their nests and eggs are protected under the Migratory Birds Convention Act, 1994 (Government of Canada 2017). Canada Warbler is also protected under Canada’s Species at Risk Act as a Threatened species (Government of Canada 2019b). The species is listed as Endangered under the Nova Scotia Endangered Species Act, Threatened under Manitoba’s Endangered Species and Ecosystems Act and the New Brunswick Endangered Species Act, and Special Concern under Ontario’s Endangered Species Act. In Quebec, Canada Warbler is included on the list of wildlife species likely to be designated as threatened or vulnerable (Gouvernement du Quebec 2018).

Non-legal status and ranks

Canada Warbler is regarded as a Sensitive species in Alberta (AESRD and ACA 2014). In British Columbia, it is considered Vulnerable (blue-listed; BC Conservation Data Centre 2019), because of uncertainty about the effects of forest harvesting on the quality of its habitat (Cooper et al. 1997).

NatureServe (2020) ranks Canada Warbler as globally secure (G5, Table 5) and the IUCN ranks it as Least Concern (BirdLife International 2019). In Canada, NatureServe (2020) considers the species as Apparently Secure (N4B) overall; at the provincial/territorial level, it is Imperilled or Critically Imperilled only in Yukon, Northwest Territories, and Prince Edward Island, all of which are at the edge of the species’ distribution and have very small populations, but is considered Vulnerable or Vulnerable to Apparently Secure in an additional six provinces (Table 5). It is one of 10 species of high conservation concern for forest managers in commercial forest in Alberta (Hannon et al. 2004).

^a G = Global; N = National; S = Subnational; B = Breeding; 1 = Critically Imperilled; 2 = Imperilled; 3 = Vulnerable; 4 = Apparently Secure; 5 = Secure; U = Unrankable (due to lack of information or conflicting information).

In the United States, Canada Warbler is considered to be a Species of Management Concern (U.S. Fish and Wildlife Service 2011). NatureServe (2020) considers its overall status to be Secure (Table 5), though Critically Imperilled or Imperilled in four U.S. states (Illinois, Indiana, Ohio, and Oklahoma).

Partners in Flight lists Canada Warbler as a species of continental importance in Canada and the United States, and a species of high conservation concern in the Northern Forest region (Rosenberg et al. 2016). Launched in 2013, the Canada Warbler International Conservation Initiative (CWICI) is a public-private partnership to improve the population status of Canada Warbler throughout its breeding and wintering ranges (Rosenberg et al. 2016). CWICI brings together representatives from the forest industry, First Nations, academia, and government and non-government agencies ranging from North America to South America, to undertake applied research priorities, and to design national and international recovery plans.

On the wintering grounds in the Andes, Finch and Stangel (1993) considered Canada Warbler as a vulnerable species due to extensive deforestation.

Habitat protection and ownership

Canada Warbler is known to occur in at least 25 of Canada’s national parks (Pruss pers. comm. 2019) and in many other protected sites under provincial and territorial jurisdiction. The total area of these protected sites is a small proportion of the species’ breeding range. Notable regional examples of large protected sites supporting Canada Warbler include: Nahanni National Park Reserve in Northwest Territories; Wood Buffalo National Park in Alberta; Prince Albert National Park and Lac La Ronge Provincial Park in Saskatchewan; Riding Mountain National Park in Manitoba; Wabakimi Provincial Park, Woodland Caribou Provincial Park, Algonquin Provincial Park, Pukaswka National Park, and Georgian Bay Islands National Park in Ontario; La Mauricie National Park in Quebec; Fundy National Park in New Brunswick; Kejimkujik National Park in Nova Scotia; and Prince Edward Island National Park in Prince Edward Island.

Acknowledgements and authorities contacted

Acknowledgements

Funding for this report was provided by Environment and Climate Change Canada (ECCC), with Annie Emard, Andrea Clouston, Karen Timm, Sydney Allen, and especially Marie-France Noel providing administrative oversight. Rosana Nobre Sores (ECCC) helped secure some data sets. Special thanks to Adam Smith, who made available unpublished data on Canada Warbler trends, to Ana Maria Gonzalez-Prieto, who supplied unpublished data on the species’ ecology on its wintering grounds in Colombia, and to Scott Wilson for providing valuable assistance with summarizing recent habitat trends on the wintering grounds. Robin Gutsell and Jocelyn Gregoire helped track down some reference materials from western Canada. Michel Robert and Alaine Camfield also provided important information, as did members of the Canada Warbler International Conservation Initiative.

This document built upon a previous status report written by Carl Savignac. Greatly appreciated were review comments provided by Christian Artuso, Bruce Bennett, Louise Blight, Ruben Boles, John Brett, Mike Burrell, Bruno Drolet, Richard Elliot, Marcel Gahbauer, Krista de Groot, Sam Haché, Carolyn Hann, Inge-Jean Hansen, Colin Jones, Dana Kinsman, Dan Kraus, Del Meidinger, Andrea Norris, Amélie Roberto-Charron, Mary Sabine, Jean-Pierre Savard, Jim Saunders, Junior Tremblay, Dave Toews, Lauren Trute, Steven Van Wilgenburg, Russell Weeber, and Liana Zanette. Thanks to Glenn Bartley for permission to use his photo for the front cover.

Authorities contacted

Bayly, N.J. Director of Migratory Species Research, SELVA: Investigación para la Conservación en el Neotropico, Bogotá, Colombia.

Blaney, S. Executive Director and Senior Scientist, Atlantic Canada Conservation Data Centre, Sackville, New Brunswick.

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Biographical summary of report writer

For over 30 years, Jon McCracken worked as a senior manager at Bird Studies Canada (BSC) – a non-profit institution dedicated to the research, monitoring, and conservation of Canada’s birds. He graduated from the University of Western Ontario with an Honour’s Bachelor of Science in zoology in 1977. Afterwards, as a contract biologist and environmental consultant for government and non-government agencies, he subsequently worked on a large variety of projects, with a particular focus on site and regional assessments of breeding birds. After joining BSC in 1989, he was heavily involved in developing, implementing and overseeing a variety of volunteer-based, bird monitoring programs, including the Canadian Migration Monitoring Network, the Marsh Monitoring Program, and the Ontario Breeding Bird Atlas. He was also extensively involved in a large variety of species at risk assessments and species recovery programs, including authoring several COSEWIC status reports, and served as co-chair of COSEWIC’s Bird Specialist Sub-committee for eight years.

Collections examined

No museum specimens were examined in the preparation of this status report.

Appendix 1. IUCN threats calculator for the Canada Warbler

Threats assessment worksheet

Species or ecosystem scientific name:

Canada Warbler

Element ID:

Not applicable

Elcode:

Not applicable

Date:

05/06/2019

Assessor(s):

Jon McCracken (report writer), Marcel Gahbauer (co-chair), Dwayne Lepitzki (facilitator), Sydney Allen, Mike Burrell, Alaine Camfield, Ana Maria Gonzalez, Robin Gutsell, Thomas Jung, Piia Kukka, Marie-France Noel, Leah Ramsay, Rich Russell, Mary Sabine, Donald Sam, Cindy Staicer, David Toews, Greg Wilson, Marc-Andre Villard

References:

Not applicable

Assigned overall threat impact:

B = High

Impact adjustment reasons:

Not applicable

Overall threat comments:

Overall, threats on the wintering grounds have potential to affect a large proportion of the population, and tend to have greater severity than those on the breeding grounds, because of the limited availability of alternate habitat. There is some overlap between scope of agriculture and logging on wintering grounds, but even if only one of these threats was scored, the overall score would remain high because of the number of other threats scored as low or medium. However, considering this overlap and the recent increase in population size despite these ongoing threats, the impact over the next ten years is anticipated to be toward the low end of the 10-70% range predicted by an overall high impact.