Least Bittern (Botaurus exilis): COSEWIC assessment and status report 2024

Official title: COSEWIC Assessment and Status Report on the Least Bittern (Botaurus exilis) in Canada

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

Special Concern

2024

COSEWIC assessment summary

Assessment summary –November 2024

Common name

Least Bittern

Scientific name

Botaurus exilis

Status

Special Concern

Reason for designation

This small, secretive member of the heron family relies on semi-open wetlands with emergent plants and relatively stable water levels. In Canada, the centre of its range is in southern Ontario and Quebec. Population size is estimated at 4300 mature individuals, an increase since the previous assessment which likely reflects improved survey coverage and techniques rather than an increase in numbers. Ongoing regional surveys over the last 10 years suggest that the Canadian population is currently stable. Nonetheless, given its specialized habitat requirements and low numbers, the species is vulnerable to future declines in Great Lakes water levels, loss of non-breeding habitat, and habitat conversion by invasive plants; it may again become Threatened if threats are not effectively mitigated or managed.

Occurrence

Saskatchewan (may occasionally breed), Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, and Prince Edward Island

Status history

Designated Special Concern in April 1988. Status re-examined and confirmed in April 1999. Status re-examined and designated Threatened in November 2001 and in April 2009. Status re-examined and designated Special Concern in November 2024.

COSEWIC executive summary

Least Bittern

Botaurus exilis

Wildlife species description and significance

The Least Bittern (Botaurus exilis) is the smallest heron in the Western Hemisphere. In shape and secretive habits, it resembles the more familiar American Bittern, but it is much smaller and somewhat more colourful. Its contrasting dark crown and back and buff wing patches distinguish it from all other marsh birds. Even so, it is very secretive and most often detected only by its cuckoo-like (coo-coo-coo) call. The Least Bittern is highly valued by naturalists as a mysterious and attractive bird representative of pristine expanses of marshland. It is considered a useful indicator of wetland health.

Aboriginal (Indigenous) knowledge

All species are significant and are interconnected and interrelated. There is no species-specific Indigenous knowledge in the report. The Least Bittern is known as Zaawmooshk'osi in Anishinaabemowin.

Distribution

The species nests from southern Canada to southern South America, with North American birds assumed to be wintering mainly along the Gulf and Mexican coasts, south to Panama. In Canada, it breeds in southern Manitoba, Ontario, Quebec, New Brunswick, and probably at least occasionally in Prince Edward Island, Nova Scotia, and eastern Saskatchewan. The majority of birds breed in southern Ontario. The Canadian range of the Least Bittern is <5% of the global range by area.

Habitat

The Least Bittern breeds strictly in marshes of emergent wetland plant species (usually cattails, Typha spp.) that have relatively stable water levels and interspersed areas of open water (that is, hemi-marsh). Such marshes have declined considerably across the species range since European settlement, although recently the rate of decline might be slowing, thanks to protection and stewardship programs for wetlands.

Biology

Adults arrive on Canadian breeding grounds from late March to early May, with calling and nesting beginning by early May. They are only weakly territorial, sometimes nesting in small, loose colonies. Thus, territory and home range size are highly variable. Nest success also varies considerably, as nests are subject to flooding, collapse, and depredation by a variety of predators. Some individuals can raise two broods in one season, but many aspects of demography remain poorly understood. Generation length is estimated to be 3.1 years. The Least Bittern is extremely secretive and feeds primarily on small fish and insects.

Population sizes and trends

About 2,150 pairs (4,300 mature individuals) are thought to nest in Canada, but the precision of this estimate is uncertain due to the species’ secretive nature. The increase in the estimated population size from the previous status report (1,500 pairs, 2009) represents an increase in search effort rather than a true increase. Historically, numbers have declined in Canada and in the northern and central United States. More recent evidence for increasing or declining population trends is more equivocal. Breeding Bird Survey (BBS) data and eBird models suggest a decline over the last 10-year period, but the trends are not significant. Marsh bird monitoring programs suggest increases in abundance in monitored regions due to temporary increases in water levels, particularly in the past five years; however, these trends are not significant, except in one region of Ontario.

Threats

Habitat loss and degradation are by far the biggest threats to the species. Historically, these threats have included wholesale destruction of marshes, mainly for agriculture, although this trend has slowed. Degradation of wetlands continues in much of the range through such factors as fragmentation, reduced water quality, and invasive marsh plants. Other threats occur more locally, such as collisions with towers, fences, and cars; recreational activities; and perhaps toxic substances such as pesticides. Threats are assumed to affect the species throughout its range, on migration, breeding, and non-breeding grounds.

Protection, status and recovery activities

The species is protected federally under the Migratory Birds Convention Act 1994 and by its current listing as Threatened under the Species at Risk Act. It is also provincially listed as Endangered in Manitoba, Threatened in Ontario, Vulnerable in Quebec, and Threatened in Nova Scotia and New Brunswick. NatureServe lists the Least Bittern as Apparently Secure nationally (N4B) and in Ontario (S4B), Imperilled in Manitoba (S2S3B) and Quebec (S2B), and Critically Imperilled in New Brunswick (S1S2B). It is classified globally as Least Concern on the IUCN Red List of Threatened Species. The Least Bittern is not listed federally in the United States, but in states that border Canada its status ranges from Not Listed to Endangered.

Technical summary

Botaurus exilis

Least Bittern

Petit Blongios

Zaawmooshk'osi (Anishinaabemowin)

Range of occurrence in Canada (province/territory/ocean): Saskatchewan (may occasionally breed), Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, and Prince Edward Island (may occasionally breed).

Demographic information:

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

Approximately 3.1 years.

Based on Bird et al. (2020).

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

Uncertain. Point estimates of change from the Breeding Bird Survey (BBS) and eBird are negative, but non-significant. Point estimates from regional monitoring are mostly positive and non-significant.

Reliability of 10-year BBS trends is Low for this species. Despite its regional nature, the Great Lakes Marsh Monitoring Program (MMP) is likely more appropriate, and its Reliability = High. MMP largely shows non-significant increases at regional levels, but Ontario BCR 13 shows a significant 10-year increase.

[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]

Unknown.

[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]

Unknown.

[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 up to a maximum of 100 years].

Uncertain. Total reduction over 10 years (2012 to 2022) estimated as 7.24% (95% CIs: -45.79, 54.11) and 4.9% (80% CIs: 0.9,

-17.1), but trends are non-significant. Conversely, regional 10-year trends from the MMP show non-significant increases over the same period (2012 to 2022), with the exception of Ontario BCR 13 (significant 43.79% increase). Ontario breeding bird atlas surveys (2021 to 2024) suggest a stable population in that province. Overall, population is likely stable.

Reductions from Canadian Breeding Bird Survey (BBS) and eBird trend analyses (2012 to 2022). Reliability of BBS trend is Low; eBird trend reliability is unranked. MMP and atlases likely more reliable for monitoring this species, but surveys only conducted at a regional scale (albeit in the species’ core Canadian range).

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

Unknown

Rate of future decline uncertain; range of % decline over next 3 generations, based on calculated threat impact of Medium – Low, includes 0 (0 to 30%). Individuals respond strongly to water levels, which have recently been beneficial in the Great Lakes; however, there are conflicting projections for water levels over the next 10 years.

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

Unknown

May see future decline, given overall threat impact of Medium – Low, but past trend is uncertain (Box 5).

Are the causes of the decline clearly reversible?

In part

Based on the threats calculator, not all causes can be reversed.

Area the causes of the decline clearly understood?

Yes

Habitat loss and degradation are the major causes of past declines.

Are the causes of the decline clearly ceased?

No

Are there extreme fluctuations in number of mature individuals?

No

Does not meet definition of extreme fluctuations.

Extent and occupancy information:

Estimated extent of occurrence (EOO)

2,278,035 km²

Calculated based on minimum convex polygon around known occurrences in the breeding season (2013 to 2024).

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

6,328 km²

Based on number of 2 x 2 km grid cells occupied in the breeding range (2013 to 2024)

Is the population “severely fragmented” that is, is >50% of its total area of occupancy in habitat patches that are (a) smaller than would be required to support a viable population, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. No
2. No

Individuals in Manitoba and Saskatchewan are separated from those in eastern Canada, but do not meet the definition of severe fragmentation

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

At least dozens, if not more

There are 250 to 500 breeding wetlands in Canada, according to survey data. Threats such as dams and water management, or climate change can have relatively uniform effects across multiple wetlands, but threat impacts will vary regionally and thus do not reduce number of “locations” below thresholds.

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

No

Based on survey data

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

No

Based on survey data

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

N/A

No recognized sub-populations

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

No

Based on survey data

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

Yes, observed and inferred decline

Habitat loss still occurring, and habitat quality is continuing to decline, with causes including the spread of invasive aquatic plant species in the Great Lakes. This spread converts wetlands into habitat unusable by or less suitable for bitterns.

Are there extreme fluctuations in number of subpopulations?

N/A

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 whichever is longer up to a maximum of 100 years, or 10% within 100 years]?

Unknown

Analysis not conducted

Threats:

Was a threats calculator completed for this species?

Yes (See Appendix 1)

Overall assigned threat impact: Medium - Low (2023)

Eight low impact threats were identified:

Residential and commercial development (IUCN 1) - Low

Agriculture and aquaculture (IUCN 2) - Low

Transportation and service corridors (IUCN 4) - Low

Human intrusions and disturbance (IUCN 6) - Low

Natural system modifications (IUCN 7) - Low

Invasive and other problematic species and genes (IUCN 8) - Low

Pollution (IUCN 9) - Low

Climate change and severe weather (IUCN 11) - Low

What limiting factors are relevant?

Availability and quality of its specific breeding habitat, most notably a low tolerance for variation in water levels during nesting.

Rescue effect (from outside Canada):

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

Declining

Neighbouring states still list Least Bittern as at risk, due to continued declines and loss of habitat

Is immigration known or possible?

Yes

Possible due to broad range of the species and perhaps due to climate change

Would immigrants be adapted to survive in Canada?

Yes

No apparent difference in habitat

Is there sufficient habitat for immigrants in Canada?

Yes

Habitat loss has slowed and species not likely inhabiting all suitable available habitat

Are conditions deteriorating in Canada?

Yes

Habitat loss has slowed somewhat vs. historical changes

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

Yes

Likely due to habitat loss in neighbouring states and throughout U.S. range

Is the Canadian population considered to be a sink?

No

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

No

Potential source populations generally small and declining

Wildlife species with sensitive occurrence data (general caution for consideration):

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

No

Most habitats are hard to access, and access to occurrence data poses a low risk to the species

Status history:

COSEWIC

Designated Special Concern in April 1988. Status re-examined and confirmed in April 1999. Status re-examined and designated Threatened in November 2001 and in April 2009. Status re-examined and designated Special Concern in November 2024.

Status and reasons for designation:

Status:

Special Concern

Alpha-numeric codes:

NA

Reason for change of status

1ii

Reasons for designation:

This small, secretive member of the heron family relies on semi-open wetlands with emergent plants and relatively stable water levels. In Canada, the centre of its range is in southern Ontario and Quebec. Population size is estimated at 4300 mature individuals, an increase since the previous assessment which likely reflects improved survey coverage and techniques rather than an increase in numbers. Ongoing regional surveys over the last 10 years suggest that the Canadian population is currently stable. Nonetheless, given its specialized habitat requirements and low numbers, the species is vulnerable to future declines in Great Lakes water levels, loss of non-breeding habitat, and habitat conversion by invasive plants; it may again become Threatened if threats are not effectively mitigated or managed.

Applicability of criteria:

A: Decline in total number of mature individuals:

Not applicable.

Evidence for a decline is equivocal. One monitoring program shows a decline in number of mature individuals over the past 10 years but point estimate is non-significant and does not meet the threshold for Threatened. The suspected trend over the next 10 years does not meet that threshold either. Regional surveys point to a stable population or an increase.

B: Small distribution range and decline or fluctuation

Not applicable.

Does not meet thresholds for EOO and IAO.

C: Small and declining number of mature individuals

Not applicable.

Estimated number of mature individuals (4,300) is below the threshold for Threatened and all individuals are in a single subpopulation. However, evidence for a continuing decline is equivocal and there are no extreme fluctuations in the number of mature individuals.

D: Very small or restricted population

Not applicable.

Estimate of 4,300 (4,240 to 4,440) mature individuals is above the threshold for D1, and the population is not vulnerable to rapid and substantial decline.

E: Quantitative analysis

Not applicable.

Analysis not conducted.

The Least Bittern meets the definition of a wildlife species of Special Concern: it has very restricted habitat (it requires specific wetland types and optimal water levels) for which a threat has been identified, and it is not yet clearly secure (population size <10,000 with potentially worsening threats).

Preface

The Canadian population of the Least Bittern was initially assessed by COSEWIC as Special Concern in April 1988 and its status was re-examined and confirmed in April 1999. The species was reassessed as Threatened in November 2001, and added to Schedule 1 of the Species at Risk Act in June 2003. In the most recent update, in April 2009, COSEWIC reconfirmed its Threatened status. The species was previously assessed under the name Ixobrychus exilis. However, in 2024, on the basis of molecular phylogenetic work by Hruska et al. (2023), the American Ornithological Society merged Ixobrychus into Botaurus. As a result, the species’ scientific name is now Botaurus exilis (Chesser et al. 2024; Poole et al. 2024).

The earlier reported declines in the Canadian Least Bittern population, which are referenced in the previous status report (COSEWIC 2009), appear to have slowed, although most national monitoring programs and analyses show non-significant trends (Fink et al. 2023; Smith et. al. 2023). Targeted surveys have also added more information on range and population size (Bartok 2011; Kirchin et al. 2020; Birds Canada 2022). Threats have remained largely unchanged, although the species appears to have benefitted from favourable water levels in the Great Lakes in more recent years.

Recovery activities for the Least Bittern in Canada since the last status report have included finalizing a national survey protocol (Jobin et al. 2011a), graduate research using targeted surveys (Bartok 2011), and the publication of a recovery strategy (Environment Canada 2014). In addition, broad efforts have been devoted to protecting, managing, and restoring wetlands in Ontario and Quebec, for example, through the Eastern Habitat Joint Venture of the North American Waterfowl Management Plan and the Great Lakes Sustainability Fund. One such habitat management project includes Phragmites australis control at Long Point, Lake Erie, Ontario. The recovery team is no longer active.

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 (2024)

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 species’ 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.

Wildlife species description and significance

Name and classification

Current classification: Class: Aves

Order: Pelecaniformes

Family: Ardeidae

Genus: Botaurus

Species: B. exilis

Subspecies in Canada: Five subspecies of Least Bittern are distinguished, based on plumage and morphology (Poole et al. 2020). Only one, Botaurus exilis exilis, breeds in Canada (see Population structure, below).

Taxonomic changes since previous report: Formerly classified in the genus Ixobrychus, but molecular phylogenetic evidence indicates that the small Ixobrychus bitterns are sister to the larger bitterns of the genus Botaurus. These small bitterns, including the Least Bittern, have therefore been moved to an expanded Botaurus (Hruska et al. 2023; Chesser et al. 2024; Poole et al. 2024). In addition, Ardeidae has been moved from Ciconiiformes to Pelecaniformes (Hackett et al. 2008).

Common names:

English: Least Bittern

French: Petit Blongios

Anishinaabemowin: Zaawmooshk'osi

Description of wildlife species

The Least Bittern is the smallest member of the heron family in the Western Hemisphere. It is one of two species in the subfamily of bitterns (Botaurinae) found in North America, the other being the much larger and more widespread American Bittern (Botaurus lentiginosus).

The Least Bittern is small for a heron, only 28 to 36 cm in length and 80 g in weight (Poole et al. 2020), not much bigger than an American Robin (Turdus migratorius). Like the American Bittern, it hunches when at rest and freezes when alarmed, standing still with its bill stretched skyward. It is brown and buffy overall, with broad buff streaks on its white underside, and a contrasting back and crown that is glossy black in adult males but lighter in females and juveniles. Buff wing patches, which are especially obvious when the bird flushes, distinguish this species from all other marsh birds.

The species is most frequently detected by its calls, either a cuckoo-like, guttural “coo-coo-coo” used in mate attraction and territorial advertisement or a rail-like “kak-kak-kak” or “ert” (Conway 2009), as well as by various other alarm, flight, and contact calls delivered singly or in short series, and given various renderings such as “ank”, “gak”, and “kuk” (Sibley 2000; Kushlan and Hancock 2005).

Designatable units

Only the nominate subspecies occurs in Canada. No significant disjunctions are believed to occur in its national distribution (the Canadian range is disjunct, but immediately to the south, the species’ range is connected through the northern U.S. states). There is currently no evidence for genetic structuring or heritable traits in discrete segments of the Canadian population. Therefore, this report deals with only one designatable unit.

Special significance

The Least Bittern is not hunted. It is highly valued by naturalists for viewing as a mysterious, attractive bird that is representative of pristine expanses of marshland. It is also considered a useful indicator of the health of such habitats.

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 Least Bittern is important to Indigenous Peoples, who recognize the interrelationships of all species within the ecosystem.

Distribution

Global range

The Least Bittern is restricted to the New World and breeds mainly in the eastern United States. (Figure 1). Its breeding range extends from southern Manitoba, Ontario, Quebec, and New Brunswick, south to the Caribbean and South America. Northern-breeding birds winter along the Atlantic coastal plain as far north as Maryland, but are primarily concentrated in Florida, and along the Gulf Coast, especially Texas, Baja California, and in the lowlands of Mexico and Central America (Poole et al. 2020).

Elsewhere in the south, the species is resident year-round. Its distribution and abundance in Mexico and Central America are poorly known (Howell and Webb 1995; Stotz et al. 1996), partly because wintering individuals are hard to distinguish from local residents (Poole et al. 2020). Previous COSEWIC assessments (1988, 1999, 2001, 2009) have implied that North American birds winter as far south as northern South America (for example, James 1999), but birds there, perhaps with rare exceptions, belong to more southerly subspecies (Restall et al. 2007). Stable isotope evidence suggests that Canadian birds winter along the Gulf Coast and coastal southeast U.S. (Bartok in prep.). Disjunct breeding populations are scattered throughout the western U.S., in Oregon, California, Arizona, New Mexico, Wyoming, and, formerly, Utah. During migration, the birds occur throughout the area between the breeding and wintering ranges. Specific migration routes are unknown (Poole et al. 2020).

Figure 1. Canadian and global range of the Least Bittern. Source: Fink et al. 2023. This material uses data from the eBird Status and Trends Project at the Cornell Lab of Ornithology, eBird.org. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Cornell Lab of Ornithology.

Canadian range

The Canadian range of the Least Bittern is <5% of the global range by area. The species is difficult to detect. Thus, information on its distribution, especially at the edges of the range, is based on relatively few detections compared to bird species that are more conspicuous and live in more accessible habitat. The Least Bittern has occurred as a vagrant in every Canadian province east of Alberta (Hudon et al. 2021), but regularly occurs and breeds only in Manitoba, Ontario, Quebec, New Brunswick, and perhaps Nova Scotia; the species may also occasionally breed in Saskatchewan (for example, Herriott 2018) and Prince Edward Island. The core of the range largely corresponds to the Temperate Wetland Region of Canada (as defined in National Wetlands Working Group 1997; Figure 2).

The northwestern limit of the known breeding range is southern Manitoba, where the species was found in 31 atlas squares of 2,994 atlas squares surveyed (Bazin 2018). Observations were most concentrated in the Interlake region; the northern part of this region, which is part of the Boreal Plains Ecozone, is considered to support relatively high numbers (Bazin pers. comm. 2022, 2023). Additional areas of higher concentration include the Rat River Swamp (south of Winnipeg) and the Brokenhead Swamp (east of Winnipeg). Other records are primarily scattered individual observations from the southwest (Carberry, Whitewater Lake, and near Virden), and along the west side of Lake Manitoba (Figure 3). Since the Manitoba atlas was published, additional records have been reported in other wetland complexes in southeastern Manitoba close to Gardenton and Caliento along the U.S. border(Poole pers. comm. 2023).

Figure 2. Canadian EOO of Least Bittern based on breeding season occurrences in NatureCounts database, April 1 to July 31, 2013-2024 (Ethier pers. comm. 2024). Points within the Boreal Shield, where wetland characteristics tend to differ from elsewhere in the species’ range, may be isolated occurrences or instead due to low survey effort (Bazin pers. comm. 2023).

Figure 3. Breeding distribution of the Least Bittern in Manitoba, according to the latest Manitoba Breeding Bird Atlas (Bazin 2018).

Ontario supports the highest number of Least Bitterns in Canada. During the 2001 to 2005 breeding bird atlas project, the species was found in 210 of the 4,964 10-km² atlas squares surveyed (Woodliffe 2007). Apart from a few sites in northern Ontario near Fort Frances, Dryden, and east of Sault Ste. Marie, most of the breeding sites were in southern Ontario (Figures 2, 4).

In Quebec, as of 2022, the Least Bittern has been found at 224 sites compared to 138 sites in 2007 (Jobin pers. comm. 2022). It was found in 62 atlas squares in the Second Atlas of the Breeding Birds of Southern Québec (2010 to 2014; Figure 5; Robert et al. 2019). Most of the occupied sites are in southwestern Quebec, mainly in the Mixedwood Plains Ecozone, and especially along the St. Lawrence, Richelieu, and Ottawa Rivers (Figure 5; Jobin pers. comm. 2022).

Figure 4. Breeding distribution of the Least Bittern in Ontario, according to the second Ontario breeding bird atlas (Bird Studies Canada et al. 2006). Note the discontinuous distribution compared to the scale shown in Figure 2. Black dots depict 10 x 10 km squares where the Least Bittern was recorded during the first atlas (1981 to 1985) but not the second (2001 to 2005). Yellow dots depict squares where it was recorded during the second atlas but not the first.

Figure 5. Breeding distribution of the Least Bittern in Quebec, according to the latest Quebec breeding bird atlas, 2010 to 2014 (Robert et al. 2019).

During the 2006 to 2010 Maritimes breeding bird atlas project, the Least Bittern was found in 10 squares: one in western Nova Scotia (including a possible breeding record) and nine in New Brunswick (with one confirmed breeding record, the only one from this Atlas; Figure 6). In New Brunswick, apparently suitable habitat is infrequently visited by birders, particularly in the Valley Lowlands and Grand Lake Lowlands ecoregions, which contain many marshes and fens that are similar to those where the bird is found in neighbouring Maine (Hayden et al. 2005; Poole et al. 2020). The only confirmed breeding record for Nova Scotia is from Amherst Point, where the species summers irregularly (Erskine 1992).

Figure 6. Breeding distribution of the Least Bittern in New Brunswick and Nova Scotia, according to the latest Maritimes breeding bird atlas project (Stewart et al. 2015).

Population structure

Some authorities previously considered the Least Bittern to form a superspecies with the Little Bittern, I. minutus, of Europe and Africa, Yellow Bittern (I. sinensis) of Asia, and possibly the extinct Black-backed Bittern (I. novaezelandiae) of New Zealand. However, the latter are currently all recognized as distinct species, and I. minutus and I. sinensis are still retained as Ixobrychus (American Ornithologists' Union 1998; Kushlan and Hancock 2005; Hruska et al. 2023). Five subspecies of Least Bittern are distinguished on the basis of plumage and morphology (Poole et al. 2024). Only one, B. e. exilis, breeds in Canada and the adjacent United States, as well as in Baja California (Mexico) and into Central America and the Caribbean; the others are year-round residents of north-central Mexico, southern Central America, and South America (Poole et al. 2024). North American populations were once divided into western and eastern subspecies (I. e. hesperis and I. e. exilis), but closer examination showed that they overlapped in the characteristics used to distinguish them, so all Canadian birds are now included in B. e. exilis (that is, all were in I. e. exilis until recently; American Ornithologists' Union 1998; Poole et al. 2020; Poole et al. 2024). Given this species’ disjunct range in the western United States, further molecular work might well show substantial genetic differentiation at a continental level (Kushlan and Hancock 2005), although perhaps not among the more continuously distributed eastern birds that form the Canadian population. Rarely, individuals are seen with chestnut in place of the normally buff areas, a plumage variant once considered a different species, specifically Cory’s Bittern (I. e. neoxenus); however, this entity is no longer considered taxonomically valid (Poole et al. 2020).

Extent of occurrence and area of occupancy

Extent of occurrence (EOO) was initially estimated at about 1,331,000 km² in Canada by COSEWIC (2009). The updated estimate of EOO is 2,278,035 km² (2013 to 2024; see Figure 2); see Fluctuations and trends in distribution, below.

Given the relatively widespread range of the Least Bittern in Canada and the fact that breeding individuals are dispersed rather than colonial, the current index of area of occupancy (IAO) is estimated at 6,328 km², well above the ‘B’ criterion threshold (2,000 km²)for Threatened status.

Fluctuations and trends in distribution

The change in EOO identified in this report compared to the previous status report results from the use of different methods to calculate EOO rather than an actual change in distribution. There is no apparent change in IAO because IAO was unknown in the previous status report (COSEWIC 2009).

Biology and habitat use

Since the previous status report, little additional research has been done on the Least Bittern’s breeding biology, especially for the Canadian population. While the basic facts of the species’ biology are now much better substantiated, the Least Bittern’s secretive habits and relatively impenetrable habitat still make it one of North America’s most poorly known birds. The Birds of the World account (Poole et al. 2020) provides the most comprehensive overview of the biology of the species. Key elements relevant to status determination are highlighted in the subsections below, with an emphasis on Canadian primary sources.

Life cycle and reproduction

Lifespan (U.S. Department of Interior 2021) and age at maturity (Poole et al. 2020) remain unknown. The congeneric Little Bittern of Europe is thought to breed in its first year, with a few individuals staying on wintering grounds before breeding in their second year (Pezzo and Gosler 2005). The longevity record for the Little Bittern is six years, but it is based on a small sample of returns (Cramp 1977). The generation length for the Least Bittern is unknown but estimated to be approximately 3.1 years (Bird et al. 2020).

Least Bitterns arrive on the Canadian breeding grounds between late March and early May (Bazin pers. comm. 2023); however, the majority of individuals likely arrive in late April–May (COSEWIC 2009). Nests are underlain by platforms on stiff vegetation, hence the need for dense robust stands of taller emergent species (Weller 1999; Rehm and Baldassarre 2007a; Poole et al. 2020; see Habitat requirements, below). The well-concealed nests, often constructed by the male within 10 m of open water or a watercourse (COSEWIC 2009; Environment Canada 2014), are formed by bending down both live and dead vegetation and adding short stems and sticks, typically arranged in a spoke-like manner, to create a platform measuring about 15 to 20 cm in diameter and 5 to 12 cm thick (Poole et al. 2020). Nest height averages 0.3 to 0.6 m above the water (range: 0.05 to 1.22 m; COSEWIC 2009; Poole et al. 2020). Nests are sometimes clumped into loose “colonies” (Kushlan 1973; Arnold 2005; Meyer and Friis 2008), with some of them being found as close together as 10 m (Weller 1961).

Eggs are laid from mid-May to June and incubated for 17 to 20 days (Poole et al. 2020). Young are fed in the nest for two weeks and near the nest for a further one to two weeks, after which they gradually start to forage for themselves. Nesting success is highly variable. Average clutch size is four to five eggs (range 2 to 7), and young are successfully raised to the point when they can leave the nest (“fledging success”) in about 50% of nests (range 20 to 84%). Mean age at first flight (fledging) is about 29 days (Bogner and Baldassarre 2002). Double brooding is known to occur (Bogner 2001) but its frequency is unknown, because many nests found late in the season are likely to be renesting attempts following failure of the first nest (Sandilands and Campbell 1988; Meyer and Friis 2008). There is no evidence that previous years’ nest sites are reused (Poole et al. 2020); however, birds have been found in the same wetland and in the vicinity of previous years’ nests. It is unlikely that nests are reused in Canada due to winter conditions destroying nests.

Juvenile and adult survival rates are poorly known (Poole et al. 2020). However, available habitat, including suitable water levels (Jobin et al. 2009; Hohman et al. 2021), likely impact survival and productivity.

Habitat requirements

Breeding habitat

The Least Bittern nests in freshwater emergent marshes and, less commonly, along the marshy edges of lakes, ponds, bogs, rivers, streams, and ditches, and in brackish and coastal salt marshes (COSEWIC 2009; Environment Canada 2014; Poole et al. 2020). Open water is needed for foraging because the Least Bittern forages visually by ambushing prey in shallow water near marsh edges, often from platforms that are constructed of bent vegetation (Poole et al. 2020). Siltation, turbidity, or excessive eutrophication makes foraging less efficient (Poole et al. 2020). The birds nest most regularly in wetlands greater than 5 ha in size, but also use smaller marshes (Environment Canada 2014). Typically, the nest is situated over shallow water (up to a metre in depth; COSEWIC 2009) in dense, tall stands of dead and/or live emergent vegetation, primarily cattail (Typha), bulrushes (Schoenoplectus), and occasionally reed grass (Phragmites), sedges (Carex), horsetails (Equisetum), and grasses (Poaceae), or in willow (Salix), or dogwood (Cornus)shrubs (COSEWIC 2009; Jobin et al. 2011b; Environment Canada 2014; Poole et al. 2020).

Migration habitat

Habitat use during migration is poorly known, but it is presumably similar to habitat use on breeding and wintering grounds (Poole et al. 2020). In one study in Missouri, USA, secretive marsh birds, including the Least Bittern, were monitored at stopover sites during spring migration and found to be positively associated with emergent vegetation cover, but negatively associated with amount of open water. Site occupancy by the Least Bittern, specifically, was also positively associated with water depth (Webb et al. 2022).

Winter habitat

Little is known about Least Bittern behaviour or ecology during winter, but presumably habitat in the non-breeding season is similar to breeding and migration habitat (Poole et al. 2020).

Movement, migration, and dispersal

Least Bitterns head south from Canadian breeding sites from late August to late September (Sandilands 2005). Key routes and stopover sites are unknown. In the two months before this southward migration, juveniles probably disperse quite widely, as is the case for most herons (Kushlan and Hancock 2005). These birds migrate at night, as implied by documented mortality from collisions with transmission lines and barbed-wire fences (Poole et al. 2020), and although details of their migratory habits are unknown, they can probably travel considerable distances with each flight, given the Little Bittern’s ability to migrate non-stop across the Mediterranean Sea and the Sahara (Kushlan and Hancock 2005).

Interspecific interactions

Diet:

Diet has not been studied in detail, but the Least Bittern is thought to prey mainly on small vertebrates (including fish, snakes, frogs, tadpoles, salamanders, and occasionally small mammals and songbird eggs or nestlings), large insects (especially odonates and orthopterans), leeches, and crayfish. The species also eats some types of vegetation (Poole et al. 2020).

Predators and competitors:

Predators of adults include the Snapping Turtle (Chelydra serpentina) and raptors. Eggs and chicks are taken by snakes, turtles, corvids, raptors (including owls), Northern Raccoon (Procyon lotor), American Mink (Neovison vison), and other herons, including conspecifics. Eggs and chicks are also pecked by the Marsh Wren (Cistothorus palustris; Poole et al. 2020). Human settlement beside marshes has been proposed as a factor that might increase access by pets and the Northern Raccoon, and thus threaten local breeding occurrences (James 1999).

Non-predatory interspecific interactions:

The Least Bittern probably does not compete intensely with other species for food resources. Other species of heron (especially the American Bittern) and minks forage for similar prey along the same marsh edges and at the same water depths (25 to 60 cm) as the Least Bittern, but they all probably take larger prey on average (Poole et al. 2020). At least in Canada, other sympatric wading birds, such as the Virginia Rail (Rallus limicola) and the Sora (Porzana carolina), forage in shallower water.

In some marshes, Muskrats (Ondatra zibethicus) cut channels through the vegetation, thus providing the interspersed areas of open water that the species requires for foraging and nesting (Weller 1999). In one wetland in Florida, the Least Bittern preferentially nested near Boat-tailed Grackle (Quiscalus major) nests, perhaps thereby gaining some protection from predators (Post and Seals 1993). In another wetland, in Iowa, the American Coot (Fulica americana) laid eggs in two of 13 Least Bittern nests; however, given that coot eggs do not hatch within the incubation period of the Least Bittern, this type of interspecific parasitism is probably biologically insignificant (Peer 2006).

Because of their aquatic habitat, wading birds in general are particularly susceptible to oil poisoning (oil toxicosis) and diseases, including type C botulism, avian cholera, aspergillosis, sarcocystis, and avian salmonellosis (Friend 1987; Friend and Franson 1999). Two diseases of particular concern are avian influenza—found in four other heron species in the USA and in the Great Blue Heron (Ardea herodias) in Canada (National Wildlife Health Center 2006; Canadian Food Inspection Agency 2023)—and West Nile Virus, which has been found in the Least Bittern (Centers for Disease Control 2005). Parasites have not been studied in detail, although trematodes, lice, and mites have been recorded (Poole et al. 2020).

Physiological, behavioural, and other adaptations

Little is known about the physiology of the Least Bittern (Poole et al. 2020). Evidence of tolerance of human disturbance is equivocal. While the species does sometimes nest in urbanized environments, it seems to prefer to nest in the more inaccessible parts of marshes (Weller 1999; Poole et al. 2020). Individuals may habituate to boat traffic where they forage; wading birds in general may forage in areas of high boat traffic. In Florida, the Least Bittern was found in significantly higher densities along airboat trails than along canals or open grassland (Bratton 1990; Frederick et al. 1990)

Limiting factors

The main limiting factor for the Least Bittern is the availability and quality of its specific breeding habitat, most notably a low tolerance for variation in water levels during nesting. Even low levels of siltation or eutrophication can reduce the quality of foraging habitat (Jobin et al. 2009; Hohman et al. 2021).

Population sizes and trends

Data sources, methods, and uncertainties

Most of the wide-scale bird surveys that provide a source of trend information (including for COSEWIC status assessments), such as the North American Breeding Bird Survey (BBS) and the Christmas Bird Count (CBC), detect the Least Bittern too rarely to provide a good basis for calculating trends (Downes and Collins 2007). In addition, the reliability of short-term BBS trends for Canada is ranked as Low (Table 1). Nonetheless, the BBS provides a temporally and spatially broad dataset that has been used to estimate the size of the Least Bittern population and other waterbird populations (Delany and Scott 2006). The BBS is conducted by volunteers, who tally the number of birds of each species that they hear or see for three minutes at each of 50 stations regularly spaced along a 40 km roadside route. Routes are stratified by degree blocks throughout North America, with >3,500 routes in the United States and >1,000 in Canada, but relatively few of these adequately sample marsh habitat. Most routes are done by the same volunteer for a number of years, reducing variance due to observer abilities. Routes can be sampled anytime between May 28 and July 7; the peak calling periods of individual Least Bittern, which can be asynchronous and each last only about 10 days (Bogner and Baldassarre 2001), can easily be missed. Reliability of the long-term (1970 to 2022) BBS trend for Least Bittern is assessed as Medium (Table 1).

Breeding bird atlas programs provide better information than the BBS on the presence of the Least Bittern, because atlas programs employ sustained effort focused on each species likely to occur in a given area. They offer searchers the incentive and time needed to learn how to locate a particular species and to search for it repeatedly in suitable habitat. In these programs, a province or state is divided into 10 x 10 km squares, within which volunteers seek out as much breeding evidence for as many species as possible over a five-year period. The abundance of each species may be estimated subjectively or by using point counts, although the Least Bittern is so hard to detect that its occurrence may remain underestimated. Nonetheless, the five-year window and species-by-species approach yield good data about a species’ presence within each square. In addition, because atlas programs are usually repeated every 20 years, they provide information on long-term trends in distribution and abundance. Atlas projects have been carried out in Manitoba, Quebec, and the Maritimes since the previous status report, and the fourth year of Ontario’s third atlas (2021 to 2025) was completed in 2024.

Local annual fluctuations in abundance add to the difficulty of estimating trends. For example, the species was reported in only 31% of atlas squares in both the first and second Ontario breeding bird atlas projects (Woodliffe 2007), and all but two sites (of ten squares in the second atlas) changed between the first and the second Maritimes atlas (Stewart et al. 2015). This variation is characteristic of the Least Bittern and the genus (Kushlan and Hancock 2005; Poole et al. 2020), and indeed of marsh birds more generally (Remsen and Parker 1990), which often shift breeding sites between years, depending on local conditions. Water levels and vegetation density seem to explain this annual variation (Chabot et al. 2014). A relationship with water levels was shown in at least two Canadian studies: there was associated variation of 0.3 to 1.3 birds per survey route across six years in the Great Lakes Basin (Craigie et al. 2003) and of 18 to 38 pairs across three years at one Quebec site (Jobin et al. 2009).

Aside from the broad-scale data gathered in atlas projects, occurrence data are collected through targeted surveys, which use a mixture of and playback of recorded calls and active listening in suitable habitat during the time of year when males are calling the most. The only widespread, long-term program that conducts these surveys is the Marsh Monitoring Program, which started in 1995 in Ontario, 2004 in Quebec, and 2013 in the Maritimes (Birds Canada 2022). MMP volunteers visit up to eight widely spaced stations at large (> 1 ha) marshes twice between 20 May and 5 July, play a taped sequence of several marsh species (including Least Bittern) for five minutes, and listen for five more minutes. The marshes and locations of the stations surveyed are chosen by the volunteers. Therefore, most sampling takes place in the more accessible portions of the more accessible marshes, which creates a sampling bias. Also, Least Bittern detectability varies so much with the timing and frequency of surveys that modifications have been proposed to the survey methods used (Meyer et al. 2006; Rehm and Baldassarre 2007b; Tozer et al. 2007). Environment and Climate Change Canada has developed a survey protocol that specifically targets Least Bittern (Jobin et al. 2011a). The use of this protocol and its prototypes in Manitoba, Quebec, and Ontario since 2004 in many areas that were not previously surveyed has considerably improved our knowledge of the species’ distribution in those provinces (Jobin 2006; Latendresse and Jobin 2007; Jobin et al. 2007; Meyer and Friis 2008; Bartok 2011; Jobin et al. 2011b; Bazin pers. comm. 2022).

The database provided by eBird (2021), in which naturalists around the world enter records of birds they have seen or heard, has grown exponentially in recent years, providing improved information on the Least Bittern’s distribution throughout its global range (Figure 1). Data submitted to the eBird database are scrutinized and reviewed by local moderators to ensure data integrity, although moderator reviews are generally restricted to anomalous observations (for example, those that are out-of-range for a species, or involve an unusually high count). Recent trend analyses have also been generated from these datasets for a range of species, including the Least Bittern (Fink et al. 2023). Trends in cumulative change in estimated relative abundance are estimated both range-wide and for 27 km x 27 km cells, which sample the breadth of the Canadian population.

Abundance

Using BBS data, Wetlands International estimated the global Least Bittern population to be 42,700 pairs (Delany and Scott 2006). Partners in Flight uses several data sources for waterbird population estimates (although not BBS data; Panjabi et al. 2024). For the Least Bittern, it provides a more recent global estimate of 310,000 individuals, and an estimate of 130,000 for Canada and the United States combined (Rosenberg et al. 2019; Partners in Flight 2024).

The species’ abundance in Canada remains poorly estimated. The first two status reports estimated the population size to be approximately 1,000 pairs (Sandilands and Campbell 1988; James 1999), while the most recent status report (COSEWIC 2009) updated that estimate to a total population of about 1,500 pairs (range: 1,000 to 2,800). This revised number was based on increased survey effort and does not reflect a true population increase. An Ontario estimate of 555 to 2,360 pairs was derived from the first Ontario atlas project (1981 to 1985) and the Ontario Rare Bird Breeding Program (Austen et al. 1994). The species’ abundance in Ontario was found to be higher from 2017 to 2019 than from1995 to 2016 (Kirchin et al. 2020). On the basis of research conducted in 2008 and 2009, Bartok (2011 estimated that there were approximately 1,400 pairs in coastal wetlands in the Canadian lower Great Lakes. Although this number was higher than previous estimates, it too was believed to reflect increased search effort rather than an increase in population size. Elsewhere, the only more recent data are from targeted surveys started in 2004 and based on the Marsh Monitoring Program, yielding estimates of 200 pairs in Manitoba (Bazin, pers. comm. 2022), and 500 to 600 pairs in Quebec (Jobin, pers. comm. 2022). Numbers in the Maritimes are still largely unknown. The second Maritimes breeding bird atlas project (2006 to 2010) detected the Least Bittern in nine squares in New Brunswick and one in Nova Scotia; Stewart et al. (2015) suggested that approximately 20 pairs breed there. Taken together, these figures yield an estimated total population of about 4,300 (range: 4,240 to 4,440) mature individuals in Canada. Given the species’ secretive nature and the lack of survey coverage, the actual number is likely higher, although it is unlikely to be over 10,000.

Fluctuations and trends

Where data are available for this species across its global range, it is generally considered to be declining (NatureServe 2023). Population fluctuations are very difficult to ascertain for the Least Bittern due to its secretive nature and the lack of suitable survey effort. Nonetheless, with the Marsh Monitoring Program and provincial atlas projects, distribution and abundance data are now becoming more readily available in Canada, and they suggest that the Least Bittern is more common than was once thought (Woodliffe 2007; Bartok 2011; Stewart et al. 2015; Bazin 2018; Robert et al. 2019; Bazin pers. comm. 2022; Jobin pers. comm. 2022). For Ontario, an analysis of data from the Great Lakes Marsh Monitoring Program (1995 to 2019) showed an increase in Least Bittern abundance from 2017 to 2019, following a period of relative long-term stability from 1995 to 2016 (Bird Studies Canada 2019; Kirchin et al. 2020). It should be noted that these high numbers coincided with a period of increasing water levels in the Great Lakes (Tozer et al. 2023) and that Kirchin et al. (2020) could not rule out an increase in regional density due to birds from nearby regions being attracted by the high water levels. Since water levels in the Great Lakes are now declining, some experts consider it likely that the species may decline there over the next decade (Fiorino pers. comm. 2023).

For Canada as a whole, long-term trends for the Least Bittern are derived from the BBS, which is limited by small sample sizes in suitable habitat and by poor detection probability. Both over the long term (1970 to 2022) and the short term (2012 to 2022), annual indices of abundance in Canada based on BBS data have fluctuated considerably (Smith et al. 2023); however, year-to-year trends show a relatively stable population that is unlikely to meet the threshold of 30% decline over 10 years (Table 1; Figure 7; Smith et al. 2023). The point estimate of the short-term (10-year) BBS trend for Canada is -7.24%, but as its 95% CIs (-45.79, 54.11) overlap zero (Smith et. al. 2023), the true nature of the trend is uncertain. At the provincial scale, sufficient BBS data are only available to generate trend estimates for Ontario and Quebec. They show non-significant 10-year trends, of -8.89% (-47.72, 54.11) in Ontario, and -0.6% (-49.86, 95.80) in Quebec. As is the case for Canada as a whole, the reliability of these trends is assessed as Low (Table 1). The Marsh Monitoring Program, which aims to detect trends in waterbird abundance and population size, also provides 10-year trends, albeit only for subregions of these two provinces. These subregions consist of specific Great Lakes basins, and Bird Conservation Region (BCR) 13. Only Ontario’s portion of BCR 13 shows a significant 10-year trend, that is, an increase of 43.79% (95% CIs: 6.80, 93.61; Table 2; see also Kirchin et al. 2020; Tozer et al. 2023). Reliability for these trends is assessed as High.

Figure 7. Rolling 10-year (three-generation) trends for Least Bittern population change in Canada based on Breeding Bird Survey data from 1970 to 2021 (Smith et. al. 2023). The orange and red horizontal lines show the 10-year (three-generation) trends for COSEWIC threshold declines of 30% and 50%, respectively. Each point estimate represents the 10-year trend ending in a particular year. Vertical bars represent the 50% (black) and 95% (grey) credible intervals.

New trend analyses using eBird data are available for the 10-year period of 2012 to 2022 (data from 7 June to 20 July; Fink et al. 2023). These trends show declines for about two-thirds of the 27 x 27 km eBird count regions that have significant trends across the Canadian Least Bittern breeding range. The Canada-wide 10-year trend for this period is estimated at -4.9% (80% CIs: 0.9, -17.1; Fink et al. 2023), but the reliability of this analysis is unassessed.

The best long-term trend data from breeding bird atlases is from Ontario, where the Carolinian region showed a statistically significant decline in the probability of observation (after 20 hours of atlassing in a square) of 44% in the 20 years between the first and second atlas projects (1981 to 1985 and 2001 to 2005). While sample sizes in these two atlases were too small to detect statistical changes in other (more northerly) ecoregions outside the Least Bittern core range in Ontario, there were also fewer occurrences in the north during the second atlas than the first. Overall, there was a 6% decrease (from 223 to 210) in the number of squares in which the species was reported between these atlases , (Woodliffe 2007). The change corrected for greater effort spent on the second atlas project, which yielded a statistically significant decline of 32% (Appendix 3 in Cadman et al. 2007). Since the Least Bittern has very specific habitat requirements, such corrections may induce a negative bias, because increased effort in squares with no marsh habitat will not yield detections, and conversely, surveys targeting Least Bittern only target specific habitat. More recently, however, for the first four of the five years of the third Ontario breeding bird atlas (2021 through to mid-July 2024), the Least Bittern was detected in 267 squares, which amounts to an increase of approximately 25% over the previous atlas. It should be noted that the current atlas is using a marsh bird survey protocol that is driving a slight increase in Least Bittern detections. In addition, numbers are thought to be relatively stable between this and the previous Ontario atlas (Burrell pers. comm. 2024).

U.S. border states with significant numbers of Least Bittern have reported small declines or stable numbers in recent atlas projects, with all figures uncorrected for the likely greater effort in recent years and thus likely greater current abundance relative to previous estimates. Specifically, Ohio reported stable numbers from 1935 to 1965, but substantial declines in this century (Rodewald et al. 2016), Michigan reported a 6% reduction in blocks over 20 years (from 102 to 92 blocks; Chartier et al. 2011), and New York reported a 9% drop in the number of occupied squares (McGowan and Corwin 2008). In Maine, revisits to 38 known breeding sites between 1989 and 1990 and 2005 showed a 37% decline in occupancy (Hayden et al. 2005). Recent 10-year BBS trends for U.S. border states are similar in nature to those in Canada, showing negative point estimates, but 95% CIs that overlap zero (that is, trends are non-significant).

Severe fragmentation

Not applicable.

Rescue effect

The species’ Canadian range is continuous with the larger breeding range in the United States. Like other herons, the Least Bittern is highly dispersive, with juveniles wandering widely at the end of the breeding season, and adults apparently shifting breeding sites across years. Although much of the historical habitat of the Least Bittern in Canada has been converted, wetland loss has slowed and even reversed in some areas (Chow-Fraser 2006), which suggests that Canada has sufficient habitat available for immigrants. However, given that adjacent populations in the northern United States are small, declining, and in many cases listed as Threatened or Endangered at the state level, rescue potential is likely weak.

Threats

Historical, long-term, and continuing habitat trends

There is little information on trends in the specific wetland types used by Least Bitterns, but information on wetlands more generally can at least serve as an index of likely trends in the species’ habitat. On a historical scale, loss of wetlands in the Least Bittern Canadian breeding range has been dramatic. Land use practices for agriculture has caused 85% of these changes (Wiken et al. 2003). In southern Ontario, wetlands have declined by over 60% during the last two centuries (Petrie 1998), with over 80% of these losses occurring near urban centres (Wiken et al. 2003). Losses in the southwestern part of Ontario (Essex County) are estimated at 97% (Snell 1989; Chow-Fraser 2006). Similarly, in the species’ Quebec range, along the St. Lawrence River, wetlands have declined by 40% to 80% since European settlement (Jean 2002), and wetland destruction and degradation continue today (Pellerin and Poulin 2013). Estimated losses at the western and eastern ends of the species’ Canadian range are also high, at 71% for the Prairies (Ducks Unlimited Canada 2023), 65% for Atlantic coastal salt marshes (Cox 1993), and 85% for the upper Bay of Fundy (Reed and Smith 1972). Similar trends have occurred for similar reasons in the species’ breeding range in the United States, which has lost half of its original wetlands (Environmental Protection Agency 2002).

Historical wetland loss in the Atlantic and Gulf states, where the birds are assumed to winter, has been similarly dramatic. An estimated 60,000 acres (about 24,000 hectares) per year were lost between 2004 and 2009 (Dahl and Stedman 2013); however, declines have slowed more recently. Coastal wetlands have declined by an estimated 52% in Texas since European settlement (Mizell 1998), and by up to 35% in Louisiana between 1932 and 1990 (Barras et al. 2003). Many of the remaining wetlands are significantly degraded by siltation (Barras et al. 2003), which besides having harmful effects on the ecosystem as a whole, renders the habitat less suitable for foraging by visual foragers such as bitterns (Weller 1999).

In Canada, the trends over recent decades are harder to assess. A wide variety of wetland inventory schemes have been started, but they have yet to be integrated or to produce clear results (Chow-Fraser 2002; Milton and Hélie 2003; Ingram et al. 2007). Although wetlands were still in steep decline from the 1950s to the 1990s, these declines are probably starting to slow, thanks to recent protection and stewardship initiatives targeted specifically at wetlands. These initiatives include federal and provincial wildlife policies and programs, programs related to the North American Waterfowl Management Plan (for example, the Eastern Habitat Joint Venture), and numerous initiatives by non-governmental organizations such as Wildlife Habitat Canada, Ducks Unlimited Canada, the Nature Conservancy of Canada, and innumerable local conservation groups (Lynch-Stewart et al. 1999). Nonetheless, in Ontario at least, local habitat loss and degradation continue on a smaller scale, through removal of smaller marshes, fragmentation of larger ones, and various types of degradation such as contamination, siltation, and disturbance (Maynard and Wilcox 1997; Environment Canada and Environmental Protection Agency 2007). As increases in Least Bittern abundance are strongly tied to increasing water levels (Kirchin et al. 2020), it is likely that drying trends, as well as trends of loss in wetland area, will affect this species.

Current and projected future threats

The Least Bittern is vulnerable to the cumulative effects of various threats throughout its annual cycle. These 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 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 assigned threat impact for the Least Bittern is Medium–Low (modified from the calculated impact of Medium as threat assessors judged that most threat impacts were at the lower end of the Low value for each category), which corresponds to an anticipated further decline of between 0 and 30% over the next ten years (equivalent to slightly more than three generations; see Appendix 1 for details). Threats are discussed below in order according to the IUCN threat number rather than the degree of threat impact, as only a Low impact was determined for each threat category during the threat assessment. Threats with an Unknown impact are included.

IUCN 1, residential and commercial development (low threat impact):

IUCN 1.1, housing and urban areas (low threat impact)

Description of threat:

Expansion of housing and urban areas still occurs throughout the species’ range, although there is general avoidance of the higher-class wetlands where Least Bitterns are likely to be found. Typically, there is less suitable bittern habitat near commercial and industrial areas.

Scope:

As many individuals can be found away from development, the scope is considered small.

Severity:

Development is expected to cause a slight decrease in habitat or population size.

IUCN 2, agriculture and aquaculture (low threat impact):

IUCN 2.1, annual and perennial non-timber crops (low threat impact)

Description of threat:

Expansion of agricultural areas still occurs throughout the species’ range including both breeding and wintering grounds. Cattle are not known to enter suitable Least Bittern habitat, nor does it appear that aquaculture operations are present in suitable habitat.

Scope:

Loss of wetland habitat and quality degradation likely impact the species; thus, the scope is assessed as small.

Severity:

Agricultural development is expected to cause a slight reduction in habitat or population size.

IUCN 4, transportation and service corridors (low threat impact):

IUCN 4.1, roads and railroads (low threat impact);

IUCN 4.2, Utility and service lines (Low threat impact); and

IUCN 4.4, flight paths (low threat impact)

Description of threat:

Since this species flies low to the ground and migrates at night, collisions can be an important source of mortality (Environment Canada 2014; Poole et al. 2020). There are anecdotal reports of Least Bittern mortality from collisions on breeding, migration, and wintering grounds, including 16 birds killed by cars or colliding with fences in one weekend at a Louisiana refuge, regular collisions with a generator tower in Ontario including eight on a single weekend, 17 strikes by airboats at one site in Florida (3% of 607 birds flushed; Frederick et al. 1990), and nine road kills in one year along a 3.2 km section of highway in Florida (Sandilands and Campbell 1988; Smith and Dodd 2003; Poole et al. 2020). Vehicle collisions may be of greatest concern where roads intersect emergent wetlands (Bartok pers. obs. 2019); individuals have also been killed along the causeway at Long Point, Ontario (Environment Canada 2014). Power line and guy wire fatalities have also been documented (Longcore et al. 2013).

Scope:

A large portion of the Canadian Least Bittern population is likely to encounter roads, utility/service lines, communication towers, and airports at some point in their life cycle; therefore, the scope is considered large to pervasive.

Severity:

Mortality risk from roads may be sufficient to have a slight effect on the population given the potential for locally high mortality rates. Fatal collisions with utility lines and communication towers are likely less frequent, with the effect on the population expected to be slight.

IUCN 6, human intrusions and disturbance (low threat impact):

IUCN 6.1. Recreational activities (low threat impact)

Description of threat:

Disturbance from recreational activities, notably boating, has been identified as a conservation concern for herons, because it can disrupt activities such as foraging and cause nest abandonment (Kushlan and Hancock 2005). Motorized recreational boating in nearshore areas creates waves that could flood nests and erode the marsh edges where the species forages (Environment Canada 2014).

Scope:

The Least Bittern primarily uses contiguous emergent wetlands (Darrah and Krementz 2010; Bartok 2011; Poole et al. 2020), where human intrusion and disturbance may be limited; therefore, the scope is small.

Severity:

The frequency and intensity of human activities is low at most sites, with overall severity no more than the lower end of slight.

IUCN 7, natural system modifications (low threat impact):

IUCN 7.2, dams and water management/use (low threat impact)

Description of threat:

The Least Bittern has a low tolerance for variation in water levels during nesting (Jobin et al. 2009; Chabot et al. 2014; Hohman et al. 2021). Alteration of water levels through the creation and management of dams has the potential to displace individuals or cause nest failure if substantial changes occur during the breeding season.

Scope:

The proportion of the Least Bittern population exposed to dams and water management is estimated to be limited, as many of these birds occur either in coastal wetlands along the St. Lawrence River or in isolated wetlands, which have limited vulnerability to this threat (although they may be affected by changes in water levels due to climate change – see IUCN 11).

Severity:

Severity is considered slight, since displacement because of unsuitable water levels may largely lead to shifting of territories. The potential for mortality (for example, pre-fledged young drowning) during the breeding season may have a somewhat greater effect on the population.

IUCN 8, invasive and other problematic species and genes (low threat impact):

IUCN 8.1, invasive non-native/alien plants and animals (low threat impact); and

IUCN 8.5, viral/prion-induced diseases (unknown threat impact)

Description of threat:

At sites across North America, including Ontario and Quebec, several invasive plants are outcompeting the cattails in which most Least Bitterns breed. These invasive species include Purple Loosestrife (Lythrum salicaria), Reed Canary Grass (Phalaris arundinacea), Common Reed (Phragmites australis), hybrid cattail (Typha x glauca) and, especially in Quebec, Flowering Rush (Butomus umbellatus; Lavoie et al. 2003; Hudon 2004; Jobin and Robillard 2005; Jobin 2006; Tozer and Mackenzie 2019). The Least Bittern can breed in Phragmites-dominated habitat if the structure is suitable (Jobin et al. 2011b); however, it shows a positive response to Phragmites control in wetland habitats (Tozer and Mackenzie 2019). All these plant species, as well as others that do not directly compete with cattails, such as European Frog-bit (Hydrocharis morsus-ranae), encourage succession of marshes to drier habitat (Blossey et al. 2001). Hybrid cattail is particularly aggressive, and creates a monoculture with little to no open water remaining in the wetlands where it occurs (Jones pers. comm. 2023). Invasive animal species are a growing concern in the Great Lakes and in Manitoba because of their disruptive effects on ecosystem function. Specifically, Common Carp (Cyprinus carpio), a species that now breeds regularly in the southern ends of Manitoba’s larger lakes including Lake Manitoba and Lake Winnipeg (Poole pers. comm. 2023), forage in ways that remove vegetation and muddy the waters so birds cannot see their prey. Direct impacts to the Least Bittern there are undocumented but likely.

Additionally, the Least Bittern may be susceptible to devastating outbreaks of disease and parasites. An unknown disease likely wiped out the birds breeding at one Iowa marsh (Kent 1951). Outbreaks recorded for other wading birds have involved the nematode Eustrongylides, which is associated with nutrient and silt runoff, and avian salmonella, which appears to be associated with sewage effluent (P. Frederick pers. comm., cited in Gibbs et al. 1992; Soulliere et al. 2018). There is evidence in Europe (Verhagen et al. 2021; Soda et al. 2022) and Canada (National Wildlife Health Center 2006; Canadian Food Inspection Agency 2023) of herons contracting highly pathogenic avian influenza (HPAI), and members of the Ardeidae are susceptible to the disease, although there are no documented cases for the Least Bittern in Canada (Canadian Food Inspection Agency 2023). It is possible that the species’ secretive nature makes it more difficult to identify cases of mortality caused by a disease outbreak (or perhaps their non-colonial nature renders them less susceptible than more colonial heron species; Poole pers. comm. 2023).

Scope:

Invasive species and diseases are pervasive within the core of the Canadian breeding range. The scope of the threat posed by highly pathogenic avian influenza is currently unknown.

Severity:

Although the Least Bittern may be displaced by invasive species, the level of severity is predicted to be only slight, given the general availability of habitat and the species’ mobility. Severity for HPAI is currently unknown.

IUCN 9, pollution (low threat impact):

IUCN 9.2, industrial and military effluents (low threat impact);

IUCN 9.3, agricultural and forestry effluents (low threat impact); and

IUCN 9.6, excess energy (unknown threat impact)

Description of threat:

The Least Bittern is directly and indirectly susceptible to pollution, particularly wastewater, oil spills, and agricultural runoff which can impact water quality in suitable habitat. The impacts of excess energy (light and noise pollution) on the Least Bittern are unknown, but potentially pose a threat to this species. Gjerdrum et al. (2021) reported that six species of heron (though not Least Bittern) had been attracted to lights and stranded at industrial sites in Atlantic Canada.

Scope:

A limited amount of habitat and individuals are impacted by pollution. Habitats in southern Ontario and Quebec are more susceptible given their proximity to human development. The scope of this threat is unknown.

Severity:

Although the Least Bittern is indirectly susceptible to pollution, many individuals occur away from pollution sources; therefore, the severity is considered slight. The severity of this threat is unknown.

IUCN 11, climate change and severe weather (low threat impact):

IUCN 11.1, habitat shifting and alteration (unknown threat impact);

IUCN 11.2, droughts (unknown threat impact);

IUCN 11.3, temperature extremes (low threat impact); and

IUCN 11.4, Storms and flooding (Low threat impact)

Description of threat:

The abundance of breeding Least Bittern along the Great Lakes and elsewhere has been shown to be closely linked to changes in water levels (Craigie et al. 2003; Jobin et al. 2009; Chabot et al. 2014; Hohman et al. 2021). The effects of climate change on water levels over the next 10 years is uncertain, with some models projecting lower water levels (that is, habitat shifting and alteration; Mortsch et al. 2006; Soulliere et al. 2018; Environmental Protection Agency 2021), variable levels (ECCC 2022a; Lam et al. 2022; Seglenieks and Temgoua 2022), and higher ones (ECCC 2022b). Prolonged and widespread drought on the prairies may impact western portions of the Least Bittern population by reducing wetland habitat. However, the shorter-term impacts of drought are unknown, as drought can also create emergent habitat. An increase in the frequency and severity of storms can increase the risk of nest failure and mortality during the breeding season, and temperature extremes can impact the survival of juveniles.

Scope:

Climate change is likely to have pervasive effects on the Least Bittern, with broad-scale impacts across the species’ range, including effects of temperature and weather extremes on survival and productivity, and changes to habitat affecting its suitability for nesting bitterns. For the Great Lakes Basin in Ontario, a recent climate change vulnerability assessment ranked the species as Less Vulnerable, indicating that range extent should not increase or decrease substantially by 2050, though range boundaries may change (Brinker et al. 2018).

Severity:

Although the Least Bittern is susceptible to climate change effects, particularly changes in water level (Jobin et al. 2009; Chabot et al. 2014; Hohman et al. 2021), overall severity is anticipated to be slight, given the species’ mobility. The severity of temperature extremes, and storms and flooding, is predicted to be slight over the next 10 years, while the severity of habitat shifting and alteration, and droughts, is unknown.

Number of threat locations

Threats with potential to cause reductions in the Least Bittern population are concentrated on changes at individual or regional breeding wetlands. There are 250 to 500 of these in Canada, according to survey data. Key widespread threats such as dams and water management, or climate change, can have relatively uniform effects across multiple wetlands, but threat impacts will vary regionally and thus do not reduce the number of “locations” below thresholds.

Protection, status, and recovery activities

Legal protection and status

The Least Bittern has been listed as Threatened under Schedule 1 of the Species at Risk Act (2002) since June 2003. The species and its nests are also protected under the Migratory Birds Convention Act. It is provincially listed as Endangered in Manitoba, Threatened in Ontario, Vulnerable in Quebec under the Act Respecting Threatened or Vulnerable Species , where it is also afforded protection under the Act Respecting the Conservation and Development of Wildlife, and it is listed as Threatened in Nova Scotia and New Brunswick. While the Least Bittern is not listed federally in the United States under the Endangered Species Act (1973), itis considered at risk in six states neighbouring Canada: Wisconsin, Michigan, Ohio, Pennsylvania, New York, and Maine.

Many breeding sites are protected federally under the National Parks Act or the Canada Wildlife Act, and in Ontario under Ontario’s Provincial Parks and Conservation Reserves Act. Ontario’s Provincial Policy Statement offers some protection to wetlands that are considered provincially significant, albeit only when applications for development are submitted or site alteration bylaws apply. Similarly, the 2022 Manitoba Water Management Strategy calls for sustainable management of all wetlands. Manitoba has also made a commitment to no net loss of wetland benefits for higher class wetlands, which is enshrined in its Sustainable Watersheds Act. Any loss of such wetlands requires mitigation. In Quebec, the Act Respecting the Conservation of Wetlands and Bodies of Water and the Quebec Water Policy call for protection for aquatic ecosystems, including wetlands. The New Brunswick Wetlands Conservation Policy commits to no loss of provincially significant wetlands and no net loss of wetland function (Rubec and Hanson 2008). Wetlands on Crown Land are generally protected by federal and provincial policies prohibiting activities that cause a net loss of function. Wetlands are also protected by the Fisheries Act, which prohibits destruction of fish habitat. Wetlands in general may be protected by various other provincial and municipal planning regulations (reviewed in Rubec and Hanson 2008).

Non-legal status and ranks

NatureServe (2023) has assessed the Least Bittern as Apparently Secure nationally (N4B), Imperilled in Manitoba (S2S3B), Apparently Secure in Ontario (S4B), Imperilled in Quebec (S2B), and Critically Imperilled in New Brunswick (S1S2B). It is listed as Least Concern globally on the IUCN Red List of Threatened Species (Table 3). It is classified as Imperilled or Vulnerable in states neighbouring Canada’s population. The species’ Canadian range encompasses a number of Important Bird Areas and Key Biodiversity Areas (IBAs and KBAs).

Land tenure and ownership

The Canadian range of the Least Bittern is quite large and therefore encompasses many parks and protected areas.

Recovery activities

Recovery activities for the Least Bittern in Canada since the last status report have included finalizing a national survey protocol (Jobin et al. 2011a), graduate research using targeted surveys (Bartok 2011), publication of the recovery strategy (Environment Canada 2014), and the identification of Critical Habitat in national parks, Migratory Bird Sanctuaries, and National Wildlife Areas. There have been broad efforts to protect, manage, and restore wetlands in Ontario and Quebec, for example, through the Eastern Habitat Joint Venture of the North American Waterfowl Management Plan and the Great Lakes Sustainability Fund. One such habitat management project includes Phragmites control at Long Point, Lake Erie, Ontario. The recovery team is no longer active.

Acknowledgements

Many thanks to all authorities and specialists who provided background and other information for this status Report, in particular Ron Bazin (Manitoba), Benoît Jobin (Quebec), and Doug Tozer (Ontario), who are Least Bittern specialists in their respective regions (and beyond). The report also benefited from comments and revisions provided by Marcel Gahbauer and Louise Blight as the Birds SSC Co-chairs overseeing the report, and SSC reviewers Andy Horn, Tara Imlay, and Jean-Pierre Savard. Additional reviewers who contributed substantively to the report were Ron Bazin, John Brett, Syd Cannings, Benoît Jobin, Colin Jones, Dave Moore, Tim Poole, and Kathy St. Laurent. The report writer extends thanks to the atlas coordinators, Birds Canada, NatureServe, eBird contributors, and the conservation database managers for the valuable observation data. Amit Saini of the COSEWIC Secretariat helped calculate the extent of occurrence and the index of area of occupancy. Special thanks are due to writers of previous versions of the Least Bittern status report, including C.A. Campbell, A.G. Horn, R.D. James, and A. Sandilands, for laying the groundwork for this report. The report writer also thanks Danielle Ethier for generating Figure 2.

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.

Authorities contacted

Bazin, R. Conservation Coordinator. Environment and Climate Change Canada. Winnipeg, Manitoba.

Cadman, M. Songbird Biologist / Ontario Atlas Coordinator. Environment and Climate Change Canada. Ottawa, Ontario.

Dale, C. Newfoundland Atlas Coordinator. Birds Canada. St. John’s, Newfoundland.

Gahbauer, M. Wildlife Biologist, Environment and Climate Change Canada. Ottawa, Ontario.

Gauthier, I. Biologiste, Coordonnatrice provinciale des espèces fauniques menacées et vulnérables, Direction générale de la gestion de la faune et des habitats, Ministère de l’Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs. Québec, Quebec.

Henry, P. Executive Director. NatureServe. Ottawa, Ontario.

Hoar, T. Private Biological Consultant. Whitby, Ontario.

Jobin, B. Bird Biologist. Environment and Climate Change Canada. Quebec City, Quebec.

Latremouille, L. Saskatchewan Atlas Coordinator. Birds Canada. Regina, Saskatchewan.

Poussart, C. Service de la conservation de la biodiversité et des milieux humides, Direction de l’expertise sur la faune terrestre, l’herpétofaune et l’avifaune, Ministère de l’Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs. Québec, Quebec.

Smith, A. Senior Biostatistician. Canadian Wildlife Service, Environment and Climate Change Canada. Ottawa, Ontario.

Tozer, D. Director. Waterbirds and Wetlands. Birds Canada. Port Rowan, Ontario.

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Biographical summary of report writer

Nick Bartok grew up in central Ontario on the doorstep of Algonquin Provincial Park and has been birdwatching since childhood. After completing a BES at the University of Waterloo with a thesis on Trumpeter Swans in Ontario, he moved to Yuma, Arizona to conduct research on secretive marsh birds along the Colorado River, and Burrowing Owls at the Salton Sea. After three spring/summer field seasons in the desert heat and two summer/fall seasons in southern Oregon bird banding, he moved to Victoria, BC to help run the bird banding operations at Rocky Point Bird Observatory. After working for non-profit and academic organizations, as well as provincial and federal governments, he completed a M.Sc. at the University of Western Ontario and studied Least Bittern populations and habitat associations at Long Point, Ontario. After graduation, he moved with his wife to Calgary to start a professional career as a wildlife biologist for a large consulting firm. After nine years in Calgary, he and the family moved back to Ontario (Bath) in fall 2019. Besides enjoying being a husband, dad, and avid bird watcher, he is also a certified trainer level bird bander with the North American Banding Council and annually leads bird banding workshops in Belize.

Collections examined

No collections were examined for the preparation of this report.

Appendix 1. Threats calculator results for Least Bittern

Species or Ecosystem Scientific name

Least Bittern

Date:

27-3-2023

Assessor(s):

Nick Bartok, Jenny Heron, Louise Blight, Tara Imlay, Ann McKellar, Richard Elliot, Colin Jones, Paul Knaga, Ron Bazin, Benoit Jobin, John Brett, Tyler Hoar, Dave Moore, Giuseppe Fiorino

Assigned Overall threat impact:

CD = Medium - Low

Overall threat comments

Overlapping threats and most threats were scored on the lower end of the range (for example, towards the 1% end of the drop-down), may have cumulatively overscored.