COSEWIC Assessment and Update Status Report
on the
Least Bittern
Ixobrychus exilis
in Canada

Photo of a Least Bittern Ixobrychus exilis.

Threatened 2009

Committee on the Status of Endangered Wildlife in Canada

Comité sur la situation des espèces en péril au Canada

Table of contents

List of Figures

List of Tables

COSEWIC status reports are working documents used in assigning the status of wildlife species suspected of being at risk. This report may be cited as follows:

COSEWIC. 2009. COSEWIC assessment and update status report on the Least Bittern Ixobrychus exilis in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 36 pp.

Previous reports

COSEWIC 2001. COSEWIC assessment and update status report on the Least Bittern Ixobrychus exilis in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. v + 10 pp.

James, R.D. 1999. Update COSEWIC status report on the Least Bittern Ixobrychus exilis in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 1-10 pp.

Sandilands, A.P. and C.A. Campbell. 1988. COSEWIC status report on the Least Bittern Ixobrychus exilis in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 40 pp.

Production note

COSEWIC would like to acknowledge Andrew G. Horn for writing the status report on the Least Bittern Ixobrychus exilis in Canada, prepared under contract with Environment Canada, overseen and edited by Richard Cannings and Jon McCracken, Co-chairs, COSEWIC Birds Specialist Subcommittee.

For additional copies contact:

COSEWIC Secretariat
c/o Canadian Wildlife Service
Environment Canada
Ottawa, ON
K1A 0H3

Tel.: 819-953-3215
Fax: 819-994-3684

Également disponible en français sous le titre Ếvaluation et Rapport de situation du COSEPAC sur le petit blongios Ixobrychus exilis au Canada – Mise à jour.

Cover photo: Least Bittern -- Photograph by Benoit Jobin.

© Her Majesty the Queen in Right of Canada, 2009.
Catalogue No. CW69-14/409-2009E-PDF
ISBN 978-1-100-12935-8

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COSEWIC Assessment Summary

Assessment Summary – April 2009

Common name - Least Bittern
Scientific name - Ixobrychus exilis
Status - Threatened
Reason for designation - This diminutive member of the heron family has a preference for nesting near pools of open water in relatively large marshes that are dominated by cattail and other robust emergent plants. Its breeding range extends from southeastern Canada through much of the eastern U.S. Information on the population size and exact distribution of this secretive species is somewhat limited. Nevertheless, the best available evidence indicates that the population is small (about 3000 individuals) and declining (> 30% in the last 10 years), largely owing to the loss and degradation of high-quality marsh habitats across its range.
Occurrence - Manitoba, Ontario, Quebec, New Brunswick, Nove Scotia
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. Last assessment based on an update status report.

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COSEWIC Executive Summary

Least Bittern
Ixobrychus exilis

Species information

The Least Bittern, Ixobrychus exilis, is the smallest heron in the Western Hemisphere, seldom seen in its dense marsh habitat. In shape and secretive habits it resembles the more familiar American Bittern, Botaurus lentiginosus, 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 call.


The species nests from southern Canada to southern South America, with North American birds wintering mainly along the Gulf and Mexican coasts, south to Panama. In Canada, it breeds in southern Manitoba, Ontario, Quebec, New Brunswick, and probably Nova Scotia, with the majority of birds breeding in southern Ontario. The estimated extent of occurrence (EO) in Canada is 1,331,000 km². The area of occupancy (AO) is much smaller, but is currently difficult to estimate, given uncertainties in population size and distribution.


Least Bitterns breed strictly in marshes of emergents (usually cattails, Typha spp.) that have relatively stable water levels and interspersed areas of open water. Such marshes have declined considerably across the birds’ range since European settlement, although recently the rate of decline might be slowing, thanks to protection and stewardship programs for wetlands.


Adults arrive on Canadian breeding grounds starting in late April, with calling and nesting beginning by mid-May. They are only weakly territorial, sometimes nesting in small, loose colonies. As such, 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 most other key facts about demography, such as age at first breeding and generation time are uncertain.

Population sizes and trends

About 1500 pairs (3000 mature individuals) are thought to nest in Canada, but the precision of this is uncertain. Numbers seem to be stable globally, but historically they have declined in Canada, and in the northern and central United States. The degree of recent declines is hard to assess, because the birds are hard to detect, but bird atlas projects and marsh bird monitoring programs suggest a decline in Ontario of >30% over the past decade. While trend information is currently lacking for other provincial jurisdictions, the majority of the Canadian population occurs in southern Ontario.

Limiting factors and threats

Habitat loss and degradation are by far the biggest threats to the species. Historically, they consisted of wholesale destruction of marshes, mainly for agriculture. More recently, habitat loss has slowed, but degradation continues in much of the range through such factors as fragmentation, reduced water quality, and invasive marsh plants. Other threats apply more locally, such as collisions with towers, fences, and cars, recreational activities, and perhaps toxins such as pesticides.

Special significance of the species

Least Bitterns are not used commercially, but are highly valued by naturalists, as mysterious, attractive birds representative of pristine expanses of marshland. They are considered useful indicators of the health of such habitats.

Existing protection or other status designations

The species is protected under the Migratory Birds Convention Act and by its current listing as Threatened under the Species at Risk Act. It is also provincially listed as Threatened in Ontario and Likely to be Designated as Threatened or Vulnerable in Quebec. It is not listed globally by the International Union for Conservation of Nature and Natural Resources (IUCN), nor federally in the United States, but 16 states have listed it under various designations of conservation concern.


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.


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 (2009)

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)Footnotea

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)Footnoteb

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

Data Deficient (DD)Footnotec

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

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


Footnote a

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

Return to footnote a referrer

Footnote b

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

Return to footnote b referrer

Footnote c

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.

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

Return to footnote c referrer

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Species information

Name and classification

The Least Bittern (Petit Blongios; Ixobrychus exilis) is the smallest member of the heron family, Ardeidae (Order Ciconiiformes, Class Aves) in the Western Hemisphere. It is one of two species of the subfamily of bitterns (Botaurinae) that are found in North America, the other being the much larger, more common, and more familiar American Bittern (Botaurus lentiginosus). Some authorities consider the Least Bittern to form a superspecies with the Little Bittern, I. minutus, of Europe and Africa, the Yellow Bittern (I. sinensis) of Asia, and possibly the extinct Black-backed Bittern (I. novaezelandiae) of New Zealand, although these are all recognized as distinct species (American Ornithologists' Union (AOU) 1998; Kushlan and Hancock 2005).

Five subspecies of Least Bittern are distinguished, based on plumage and morphology. Only one, I. e. exilis, breeds in Canada. Four subspecies are year-round residents of Central and South America. North American populations were once divided into western and eastern subspecies (I. e. hesperis and I. e. exilis), but closer examination showed they overlapped in the characteristics used to distinguish them, so they are now all included in I. e. exilis (Gibbs et al. 1992; AOU 1998). Given this species’ disjunct range, especially in the United States (see below), further molecular work might well show substantial genetic differentiation (Kushlan and Hancock 2005), albeit perhaps not among eastern birds, which are more continuously distributed. Rarely, individuals are seen with chestnut in place of the normally buff areas, a plumage variant once considered a different species, “Cory’s Bittern”, but no longer considered taxonomically significant (Gibbs et al.1992).

Morphological description

The Least Bittern is tiny for a heron, only 30 cm in length and 80 g in weight, not much bigger than an American Robin (Turdus migratorius). Like the American Bittern, it hunches at rest and freezes when alarmed, 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. It is most frequently detected by its calls, either a cuckoo-like, guttural “cu-cu-cu” used in mate attraction and territorial advertisement, a rail-like “rick-rick-rick-rick”, or various 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).

Genetic description

The genetic structure of populations in this species has not been studied.

Designatable units

Only the nominate subspecies breeds in Canada; no significant disjunctions are believed to occur in its national distribution; and there is currently no evidence for genetic distinction in the Canadian population. Thus, this report deals with only one designatable unit.

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Global range

Least Bitterns are restricted to the New World, and breed mainly in the eastern U.S. (see Figure 1). Their breeding range extends from southern Manitoba, Ontario, Quebec, New Brunswick, and, perhaps irregularly, Nova Scotia, south to the Caribbean and South America. Birds winter along the Atlantic coastal plain south of Maryland, but mainly winter in Florida, and along the Gulf Coast, especially Texas, Baja California, and in the coastal lowlands of Mexico and Central America (Gibbs et al. 1992). Elsewhere in the south, they are resident year-round. Their 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 (Gibbs et al. 1992). Previous COSEWIC reviews have implied that North American birds winter south to northern South America (e.g., James 1999), but in fact birds there, perhaps with rare exceptions, belong to more southerly subspecies (Restall et al. 2007). Disjunct breeding populations are scattered through the western U.S., in Oregon, California, Arizona, New Mexico, Wyoming, and, formerly, Utah. During migration, birds occur throughout the area between the breeding and wintering ranges. Specific migration routes are unknown (Gibbs et al. 1992), but the pattern of migration might be similar to that of the Little Bittern, which moves along a broad front (Nankinov 1999).

Figure 1. Global range of Least Bittern (modified from NatureServe 2008).

Map showing the global range of the Least Bittern.

Canadian range

Least Bitterns are hard to detect, so information on their distribution, especially at the edges of their range, is based on relatively few detections, compared to bird species that are more conspicuous and live in more accessible habitat. Least Bitterns have occurred as vagrants in every province (James 1999; Natural History Society of Prince Edward Island (NHSPEI) 2008), but occur regularly and breed only in Manitoba, Ontario, Quebec, New Brunswick and perhaps Nova Scotia. The range largely corresponds to the Temperate Wetland Region of Canada (as defined in National Wetlands Working Group 1997), although the eastern end of the Prairie Wetland Region in Manitoba and the southern edge of the Boreal Wetland Region in Manitoba, Ontario, and possibly Nova Scotia also hold breeding birds (Figure 2).

Figure 2. Canadian range of Least Bittern according to information assembled by the Canadian Least Bittern Recovery Team, 2007. Points indicate locations isolated from the known breeding range, but where birds have been found during the breeding season since 1997 (modified from B. Jobin personal communication (pers. comm.) 2008). Point locations in western Ontario are probably isolated because of low survey effort, and thus should be included within the main range extending from Manitoba. Note that the range as shown here obscures the discontinuous distribution of breeding sites; compare with Figure 3.

Map showing the global range of the Least Bittern.

The northwestern corner of the Least Bittern’s breeding range is southern Manitoba, where the species has been found in 18 wetlands: 12 in the Interlake region and six farther south, with most birds in two especially large wetlands: Rat River Swamp and Brokenhead Swamp (Hay 2006; R. Bazin pers. comm. 2008). Significant numbers also occur in the northern part of the Interlake region, within the Interlake Plain ecoregion, which is part of the Boreal Plains ecozone (R. Bazin pers. comm.2008).

Most Least Bitterns in Canada are found in Ontario. During the 2001-2005 breeding bird atlas project, currently the best source for data on distribution in this province, they were found in 226 of the 4964 10-km² atlas squares surveyed (Woodliffe 2007). Apart from a few sites in western Ontario near Fort Frances, Dryden, and east of Sault Ste. Marie, most breeding sites were in southern Ontario. Breeding sites were particularly concentrated in the Mixedwoods Plains ecozone south of the Boreal Shield, specifically near Lake St. Clair, Long Point, and south of the shield between Peterborough and Kingston (Figure 3).

Figure 3. Breeding distribution of Least Bittern in southern Ontario, according to the latest Ontario breeding bird atlas project (from Woodliffe 2007). Note the discontinuous distribution within the main range as it is illustrated in Figure 2.

Map showing the breeding distribution of the Least Bittern in southern Ontario, according to the latest Ontario breeding bird atlas project.

In Quebec, as of 2007, Least Bitterns have been found at 138 sites, 48 of which have been discovered since more intensive searches were started by the Canadian Wildlife Service in 2004 (B. Jobin pers. comm.2008). Most sites are in southwestern Quebec, mainly in the Mixedwoods Plains ecozone, and especially along the St. Lawrence, Richelieu, and Ottawa Rivers (B. Jobin pers. comm. 2008).

No province-wide searches focused on this species have been conducted in the Maritimes. In New Brunswick, much 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 (Gibbs et al. 1992; Hayden et al. 2005). Breeding or likely breeding records come from only five sites, mostly in the southeast of the province (Figure 2); Maritime Breeding Bird Atlas 2008). The only breeding record for Nova Scotia is from Amherst Point, where the species summers irregularly (Erskine 1992). Most other records from the province are from outside the breeding season, but birds have been found at four additional sites during the breeding season, and could perhaps represent breeding pairs (Figure 2).

With the possible exception of Ontario, where atlas results showed no appreciable change in geographic distribution between 1981-1985 and 2001-2005 (Woodliffe 2007), no province has enough data to offer reliable information on distribution trends. Indeed, as noted above, in Manitoba and Quebec, even a modest level of directed search effort for this species has already extended its known distribution. Nonetheless, the species’ habitat needs (see below) likely restricted its distribution to the ecoregions where it is found today.

The extent of occurrence of Least Bitterns in Canada is 1,331,000 km² (measured as a minimum convex polygon). The area of occupancy is currently difficult to estimate, given uncertainties in population size and distribution. In addition, density and home range estimates vary considerably and home ranges often overlap (see “Habitat requirements,” below).

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Habitat requirements

Least Bitterns preferentially breed in marshes with tall emergent vegetation (usually cattails, Typha spp.), relatively stable water levels (less than 1 m, and usually 10-50 cm), and about 50% open water interspersed in small pockets throughout the vegetated areas (“hemi-marsh”; Gibbs et al. 1992; DesGranges et al. 2006; Budd 2007). Larger wetlands (> 5-10 ha) are said to be particularly important (Gibbs and Melvin 1992; Gibbs et al. 1992), with most detections among subsamples of marshes in Iowa, Maine, and Ontario occurring in marshes larger than 5 ha (Brown and Dinsmore 1986; Gibbs and Melvin 1992; Tozer 2002). Nonetheless, territorial birds have been found in marshes less than 0.5 ha in size (Gibbs et al.1992), and such marshes are widespread and poorly searched. Breeding densities range from 0.04 to 5 calling birds per ha, or 1-5 nests per ha (Gibbs et al. 1992; Arnold 2005; Winstead and King 2006). Colonial nesting can occur locally, with pockets of over 15 nests per ha, perhaps because of food abundance or limited nesting sites (Kushlan 1973; Bowyer et al. 2002; Arnold 2005; Meyer and Friis 2008). Radiotracking studies show that mean home range varies widely, with a New York study reporting 9.7 ha (n=33; Bogner 2001; Bogner and Baldassarre 2002) and a Missouri study reporting 98 ha (n=18; Griffin et al. 2006). Such large variance in both breeding density and home range size compounds the difficulty of assessing how important marsh size is for breeding.

The Least Bittern’s habitat needs are largely dictated by its nesting and foraging habits. Nests are underlain by platforms on stiff vegetation–thus the need for dense robust stands of taller emergent species–and are almost always within 10 m of open water–thus the need for interspersed open water (Gibbs et al. 1992; Weller 1999; Rehm and Baldassarre 2007a). Open water is also needed for foraging, because Least Bitterns forage visually by ambushing their prey in shallow water near marsh edges, often from platforms that they construct out of bent vegetation (Gibbs et al. 1992). Access to clear water is essential for the birds to see their prey; siltation, turbidity, or excessive eutrophication makes foraging less efficient (Gibbs et al.1992).

Although most known Canadian breeding sites are large marshes, some are other types of wetland, such as sloughs and bogs. Similarly, most breeding sites in Canada are dominated by cattails. Nevertheless, breeding also occurs in areas with other robust emergents, such as Scirpus, Phragmites, Equisetum, Butomus, Sparganium, and Carex (Sandilands and Campbell 1988; Gibbs et al.1992; Jobin and Robillard 2005; Meyer and Friis 2008), and in shrubby swamps dominated by Buttonbush (Cephalanthus occidentalis), willow (Salix spp.) or alder (Alnus spp.; Jobin 2007; Latendresse and Jobin 2007; R. Bazin pers. comm.2008). Elsewhere in the U.S. breeding range, breeding sites are dominated by other species that are physically similar to cattails, such as Sawgrass (Cladium jamaicense), Giant Cutgrass (Zizaniopsis miliacea), and Swamp Loosestrife (Decodon verticillatus; Gibbs et al. 1992; Winstead and King 2006). Thus, the structure of the vegetation–tall and dense, but not so impenetrable as to restrict movement–seems to be more important than the species (Nelson 2003; Arnold 2005).

The presence of some shrubby vegetation has been identified as a potentially important habitat variable in some regions, but not across its range. Specifically, the likelihood that particular marshes were occupied increased with shrubby vegetation in Manitoba (Hay 2006), decreased in the Great Lakes Basin (Kirk et al. 2001) and Tennessee (Winstead and King 2006), and was equivocal in Arkansas (Budd 2007). The most consistent correlate of occupancy at the landscape level is the presence of surrounding wetlands (Hay 2006; Budd 2007).

Water levels must be relatively stable throughout nesting. Adults can raise nests somewhat to deal with rising waters, but persistent or sudden increases will flood nests (Nelson 2003; Arnold 2005). Conversely, drops in water level during nesting can reduce foraging opportunities and enhance access for predators (Arnold 2005).

Nevertheless, when nests are not active, exposure to variation in water levels enhances Least Bittern habitat in the long term. Specifically, periodic drying enhances the growth of emergents, while periodic flooding arrests succession to shrubby vegetation (Arnold 2005). Artificial impoundments, when appropriately managed, can provide these needed conditions and be rapidly colonized by Least Bitterns (Jobin et al. 2009)

Needs for wintering habitat are less specific, and appear to be met by a wide variety of wetlands, not only emergent marshes like those used for breeding, but also brackish and saline swamps (Frederick et al. 1990; Gibbs et al. 1992). Habitat use during migration is poorly known, but presumably is similar to breeding and wintering habitat (Gibbs et al.1992).

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’s Canadian breeding range has been dramatic. Land use practices for agriculture 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), and losses in the southwest (Essex County) estimated at 97% (Snell 1989). Similarly, in the species’ Quebec range, along the St. Lawrence River, wetlands have declined by 80% since European settlement (Jean 2002). Estimated losses at the western and eastern ends of the species’ Canadian range are only slightly lower, at 71% for the Prairies, 65% for Atlantic coastal salt marshes (Cox 1993), and 85% for the upper Bay of Fundy (Reed and Smith 1972). Similar trends, for similar reasons, occurred in the species’ breeding range in the United States, which has lost half of its original wetlands (United States Environmental Protection Agency (USEPA) 2002).

Historical wetland loss in the Gulf States, where the birds winter, has been similarly dramatic. 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 its harmful effects on the ecosystem as a whole, renders the habitat less suitable for foraging by visual foragers such as bitterns (Weller 1999). Wetlands in Central America have been exposed to similar threats, as well as chemical contamination, but their decline has not been quantified (Davidson and Gauthier 1993).

 In Canada, more recent habitat trends (i.e., on the scale of the last few decades) are harder to assess. A wide variety of 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 initiatives for protection and stewardship targeted specifically at wetlands. These initiatives include federal and provincial wildlife policies and programs, programs related to the North American Waterfowl Management Plan (e.g., 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 continues at 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; EC and USEPA 2007; see also Threats, below). Declines in strongly wetland-dependent bird species throughout the Great Lakes Basin over the past 10 years, despite increases in some wetland edge and generalist species, also suggest recent deterioration in habitat conditions (Timmermans and Archer 2007).

In the United States, wetland loss has started to reverse, but mainly because of an increase in the abundance of freshwater ponds (Dahl 2006). Indeed, declines in freshwater emergent marshes (i.e., Least Bittern breeding habitat) continue, with a 1% decline from 1998 to 2004, albeit mainly involving marshes smaller than 2 ha (Dahl 2006), which are less likely to harbour this species (see “Habitat needs,” above). As in Canada, incremental loss and degradation is much harder to quantify but is still occurring (Dahl 2006). Loss and degradation of wetlands continues on the wintering range in the Atlantic coastal plain, Gulf Coast, and Central America (Davidson and Gauthier 1993; Barras et al. 2003; Dahl 2006). For example, 15-32% of the area of coastal wetland existing in Louisiana in 1978 was gone by 2000 (Barras et al. 2003).

Habitat protection/ownership

Many breeding sites are protected federally under the National Parks Act or Canada Wildlife Act, or provincially 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 development applications are made or site alteration bylaws apply. Similarly, the Manitoba Water Strategy calls for sustainable management of all wetlands, the Quebec Water Policy calls for some protection for aquatic ecosystems, and 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 throughout Canada are protected by federal and provincial policies prohibiting activities that yield a net loss of function, and by the Fisheries Act, which prohibits destruction of fish habitat. Wetlands in general are somewhat protected by various other provincial and municipal planning regulations (reviewed in Rubec and Hanson 2008).

The amount of habitat that is protected can only be guessed, both because the species’ pattern of occupancy is poorly known, and because the protective status of wetlands has not been fully inventoried. Wildlife Habitat Canada estimates that 9.2% of Canada’s wetlands are under protected areas designations (IUCN standards I to VI), but this estimate includes only 1.7% of marshes in the Mixedwoods Plains ecozone, the heart of this species’ distribution, and 0.2% of the Boreal Shield ecozone, the next most important ecozone for the species (Wiken et al.2004).

Larger marshes, which appear to be preferred by Least Bitterns, are more likely to receive protection than smaller marshes. Indeed, most marshes in which Least Bitterns have been reported have some type of protected status. Specifically, of 74 Ontario marshes where the Marsh Monitoring Program detected them since 1995, one third have federal or provincial designations (such as National Wildlife Areas, Wildlife Management Areas, or Parks), one third are protected as municipal Conservation Areas or under similar policies, and another third are privately managed, but virtually all have some kind of stewardship program in place. In Quebec, of the 130 sites that have been evaluated, 53 sites are entirely within protected areas (IUCN standards I to VI), four sites are partially protected, and 12 sites have protected status pending (B. Jobin pers. comm. 2008). In Manitoba, the protective status of likely breeding sites has not been tabulated, although most sites are on provincial crown land, including many that are within Wildlife Management Areas and are thus likely to receive some form of protection (R. Bazin pers. comm. 2008). In Nova Scotia and New Brunswick, at least three possible breeding sites are in Migratory Bird Sanctuaries or National Wildlife Areas (D. Amirault-Langlais pers. comm.2008). Thus according to these tallies, up to two thirds of marshes where Least Bitterns nest in Canada are in protected areas.

These totals may be highly misleading, however. Protected area status is not always effective in providing protection. Habitat loss or degradation may still occur from on-site factors such as leaching from septic systems, habitat succession, and fire suppression. Moreover, many of these marshes are only partly protected, with large portions unprotected or bordering unprotected land. Indeed, areas protected under regional or municipal policies or park designations are included as protected areas, even if those measures are relatively weak (B. Jobin, pers. comm.2008). Even National Wildlife Areas, which can offer particularly strong protection against habitat loss and degradation, are significantly threatened by construction and disturbance related to recreational use, by impacts originating off site, such as contamination and siltation, and by invasive species (A.M. Turner & Associates 2002). Most importantly, however, both formal survey programs and naturalists in general are more likely to search for Least Bitterns in larger, federally and provincially protected marshes, which of course might bias any estimation of the bird’s protected habitat.

In short, data on the local distribution and abundance of Least Bitterns are so poor, and wetland inventories are still sufficiently incomplete, that while the protective status of the bird’s habitat is likely improving, it remains unmeasured.

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Since the previous status report, the number of substantive studies of the Least Bittern’s breeding biology has dramatically increased. 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.

Life cycle and reproduction

In Canada, Least Bitterns arrive on breeding grounds from late April to late May, and by mid-May males begin the brief (c. 6 week) calling period during which they are most easily detected by marsh bird surveys. They appear to be at least somewhat territorial, because calling males and nests are usually regularly spaced, and because the birds display a variety of territorial behaviours, for example, approaching and displaying to playback of their calls (Bogner 2001; Arnold 2005). Nonetheless, three lines of evidence suggest territoriality is weak or variable: responses to playback are weak compared to other marsh nesting species (Tozer et al. 2007), radiotracking studies show that home ranges often overlap (Bogner 2001; Griffin et al. 2006), and, as already noted, nests are sometimes clumped into loose “colonies” (Kushlan 1973; Arnold 2005; Meyer and Friis 2008).

First eggs are laid from mid-May to June, and incubated for 17 to 20 days (Gibbs et al. 1992). Young are fed in the nest for two weeks and near the nest for a further one to two weeks, gradually starting to forage for themselves (Gibbs et al.1992). Nesting success is highly variable. Average clutch size is four to five eggs (range 2-7), and young are successfully raised to the point when they can leave the nest (“fledging success”) in about 50% of nests (range 20-84%). Causes of nest failure include predation, nest collapse from wave action or wind, flooding, and abandonment (Gibbs et al. 1992; Arnold 2005; Lor and Malecki 2006; Pierluissi 2006; Yocum 2007). At one site in New York, 17% of pairs raised second broods in the same season (Bogner 2001). However, the degree to which double brooding is widespread is unknown, because many nests found late in the season are likely to be renesting attempts following failure of first nests (Sandilands and Campbell 1988; Meyer and Friis 2008).

Diet has not been studied in detail, but Least Bitterns are 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, slugs, crayfish, and some vegetation (Gibbs et al. 1992).

Little is known about Least Bittern behaviour or ecology during winter. Wintering populations presumably comprise both wintering birds that bred farther north as well as local residents, but there are no firm data on the relative proportion of wintering birds and residents at any given site (Gibbs et al. 1992; Howell and Webb 1995).

Most key demographic information is lacking. Lifespan and age at maturity are unknown, although 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 Little Bitterns is six years, albeit based on a small sample of returns (Cramp 1977), and their generation time is unknown but estimated to be less than 3.3 years (BirdLife International 2007).


Predators on adults include snapping turtles and raptors, with eggs and chicks being taken by snakes, turtles, corvids, raptors (including owls), Raccoons (Procyon lotor), American Mink (Neovison vison), and other herons, including conspecifics. Eggs and chicks are also pecked by Marsh Wrens (Cistothorus palustris; Gibbs et al. 1992). Human settlement beside marshes has been proposed as a factor that might increase access by pets and Raccoons, and thus threaten local breeding populations (James 1999).


Little is known of the physiology of Least Bitterns (Gibbs et al. 1992), although one factor relevant to their viability is a likely high exposure to toxins and disease (see below).


Birds head south from Canadian breeding sites from late August to late September (Sandilands 2005). In the two months before then, juveniles probably disperse quite widely, as in most herons (Kushlan and Hancock 2005). They migrate at night, as indicated by nocturnal tower kills (Gibbs et al. 1992), 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

Least Bitterns probably do not compete intensely with other species for food resources. Other species of heron (especially American Bitterns) and mink forage for similar prey along the same marsh edges, and at the same water depths (25-60 cm), as Least Bitterns, but they all probably take larger prey on average (Gibbs et al. 1992). At least in Canada, other sympatric wading birds, such as Virginia Rail (Rallus limicola) and Sora (Porzana carolina), forage in shallower water.

In some marshes, Muskrats (Ondatra zibethicus) cut channels through the vegetation, thus providing the interspersion of open water that the species requires for foraging and nesting (Weller 1999; see “Habitat requirements,” above). In one wetland in Florida, Least Bitterns preferentially nested near the nests of Boat-tailed Grackles, Quiscalus major, perhaps thereby gaining some protection from predators (Post and Seals 1993). In another wetland, in Iowa, American Coots, Fulica americana, laid eggs in 2 of 13 nests, but given that coot eggs do not hatch within the incubation period of bitterns, this type of interspecific parasitism is probably biologically insignificant (Peer 2006).

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 recently are H5N1 avian influenza, which has been found in four other heron species (National Wildlife Heath Center 2006) and West Nile Virus, which has been found in Least Bitterns (Centers for Disease Control 2005). Parasites have not been studied in detail, although trematodes, lice, and mites have been recorded (Gibbs et al. 1992).


Least Bitterns have specific habitat requirements, detailed in “Habitat requirements”, above. As detailed there, they have a low tolerance for variation in water levels during nesting, and even low levels of siltation or eutrophication can reduce the quality of foraging habitat. Evidence of their tolerance of human disturbance is equivocal. Specifically, while they do nest in urbanized environments and seem to habituate to boat traffic where they forage, they also seem to prefer to nest in the more inaccessible parts of marshes (Gibbs et al. 1992; Weller 1999).

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Population sizes and trends

Search effort

Most of the wide-scale bird surveys that provide information for most other bird species, such as the North American Breeding Bird Survey and the Christmas Bird Count, detect Least Bitterns too rarely to be of any use for calculating trends (Downes and Collins 2007; Sauer et al. 2007). Nonetheless, because Breeding Bird Surveys have been used to estimate global and North American populations (Delany and Scott 2006), they are briefly described here. The survey is conducted by volunteers, who tally the number of birds of each species that they hear during three minutes, at each of 50 stations regularly spaced along a 40 km roadside route. Routes are randomly placed throughout North America, with over 2300 routes in the United States and over 350 in Canada. Most routes, however, do not adequately sample marsh habitats. Most routes are ideally done by the same volunteer for years, thus reducing variance due to observer abilities and search effort. Routes can be sampled any time between May 28 and July 7, so the peak calling periods of individual Least Bitterns, which can be asynchronous and each last only about 10 days (Bogner and Baldassarre 2001), can be easily missed.

Breeding bird atlas programs arguably provide better information on the presence of Least Bitterns than Breeding Bird Surveys do, because atlas programs employ sustained effort focused on each species likely to occur in a given area. They thus offer searchers the incentive and time needed to learn how to locate 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 using point counts, although Least Bitterns are so hard to detect that, for this species, these estimates are unreliable. Nonetheless, the five-year window and species-by-species approach yield fairly good data about whether the species is present within each square. Also, because atlas programs are usually repeated every 20 years, they provide some information on trends in distribution and abundance.

Otherwise, assessment of populations must come from targeted surveys, which use a mixture of listening and playback of recorded calls, in suitable habitat during the time of year when males are calling the most. The only widespread, long-term program to do so is the Marsh Monitoring Program, which started in 1995 in Ontario, and 2004 in Quebec (Bird Studies Canada 2003). Here, volunteers visit up to eight widely spaced stations at large (> 1 ha) marshes twice between May 20 and July 5, 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 stations surveyed are chosen by the volunteers. Thus, most sampling is in the more accessible portions of more accessible marshes, which creates a sampling bias. Also, Least Bittern detectability varies so much with the timing and frequency of surveys that modifications of these surveys have been proposed (Meyer et al. 2006; Rehm and Baldassarre 2007b; Tozer et al.2007). Such a protocol that specifically targets Least Bitterns has been developed by Environment Canada and the recovery team. Application of 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; R. Bazin pers. comm.2008; Meyer and Friis 2008).


The most authoritative estimate of the global population uses Breeding Bird Survey abundance data, corrected for detectability and time of day (Rosenberg and Blancher 2005), and extrapolates them to the estimated area of occupancy (Delany and Scott 2006). This estimate yields 42,700 pairs, or 128,100 individuals of the subspecies I. e. exilis worldwide (Delany and Scott 2006; their ratio of three individuals to one pair accounts for immature birds, following Meininger et al. 1995). As noted above, however, the Breeding Bird Survey methodology is extremely poor at detecting this species, and the method of extrapolation is subject to many caveats (Thogmartin et al. 2006), so this estimate should be treated with considerable caution.

As for previous status reports, abundance in Canada is still unknown, although some new information leads to a slight change in the estimate of approximately 1000 pairs given in previous status reports (Sandilands and Campbell 1988; James 1999). An Ontario estimate of 555-2360 pairs was derived from the first Ontario atlas project and the Ontario Rare Bird Breeding Program (Austen et al. 1994). Since then, because standard methods detect this species so unreliably, the only new population estimates that have appeared come from targeted surveys by the Least Bittern Recovery Team since 2004, which have increased the Manitoba estimate from fewer than 100 pairs to up to 200 pairs (R. Bazin, pers. comm. 2008), and have increased the Quebec total from 100 pairs to 200-300 pairs (B. Jobin, unpubl. data). These do not represent increases in population size, but reflect increased search efforts. Surveys using these protocols are just starting in Ontario, so there is no refined population estimate for the province. Thus, the best available information comes from the latest Ontario atlas project, which reported that sightings on standard point counts were too scarce to provide an abundance estimate, although the frequency of occurrence within atlas squares was similar to or only slightly lower than in the previous atlas project (Woodliffe 2007; see below). Numbers in the Maritimes are unknown. Given the small number of sites with confirmed or suspected breeding, there may be fewer than 20 pairs (J. Stewart, pers. comm.2008), but the Maritimes Breeding Bird Atlas project, now underway, should provide a better estimate.

Taken together, these figures yield an estimated total population of about 1500 (between about 1000 and 2800) pairs in Canada. The certainty of this estimate is obviously poor, although it is somewhat corroborated by the global estimate quoted earlier (Delany and Scott 2006). Specifically, multiplying the global estimate (42,700 pairs) by Canada’s proportion of the species range (2-3%) yields an estimated Canadian population of 850-1300 pairs.

Fluctuations and trends

Local annual fluctuations in abundance add to the difficulty of estimating long-term trends. For example, there was a 79% turnover of atlas squares reporting the species between the first and second Ontario breeding bird atlas projects (Woodliffe 2007). This variation is characteristic of this species and genus (Gibbs et al. 1992; Kushlan and Hancock 2005), and indeed of marsh birds more generally (Remsen and Parker 1990), which often shift breeding sites between years, depending on local conditions. Water levels, in particular, seem to explain annual variation in at least two Canadian studies: variation of 0.3-1.3 birds per survey route across six years in the Great Lakes Basin (Craigie et al. 2003) and of 18-38 pairs across three years at one Quebec site (Jobin et al. 2009).

Global trends are unknown (Delany and Scott 2006), but enough information exists to suggest that the population decline criterion of the IUCN Red List (> 30% in 10 years or three generations) has not been met (BirdLife International 2004). In North America, the consensus of all previous reviews is that Least Bitterns have declined historically across their breeding range except perhaps the Gulf States (Gibbs et al. 1992; James 1999; Kushlan and Hancock 2005). This conclusion is partly based on overall habitat declines (see “Habitat Trends,” above), but also on historical (often anecdotal) reports of Least Bittern abundance in areas where they are now rare or absent. In Ontario, the heart of their Canadian range, they were considered abundant at the turn of the last century, uncommon by the 1930s, and rare by the 1980s (Sandilands 2005; Woodliffe 2007). Locally, there were fewer than two pairs by the 1980s at Hamilton Bay and Toronto, where they were common to abundant c. 1900; they appeared to decline substantially in the 1990s at Long Point, where they were common as recently as the 1980s (Austen et al. 1994); and no nests have been reported from Point Pelee since 1981, where 25 nests were reported by a single observer as recently as the 1970s (Wormington 2006).

Recent trends are arguably best measured by breeding bird atlas programs, which use representative, region-wide sampling, and require observers to focus on each species likely to be found in their local area (see “Search effort,” above). In Ontario, the Carolinian region showed a statistically significant decline in the probability of observation (after 20 hours of atlasing in a square) of 44% in the 20 years between the first and second atlas projects (1981-85 and 2001-2005). While sample sizes were too small for detecting statistical changes in other (more northerly) ecoregions outside the Least Bittern’s core range in Ontario, there were also fewer occurrences in the north during the second atlas. Overall, the number of squares reporting the species decreased by 6%, from 223 to 210 (Woodliffe 2007). The change corrected for greater effort spent on the second atlas project, however, yields a statistically significant decline of 32% (Appendix 3 in Cadman et al. 2007). Border states with significant numbers of Least Bitterns reported small declines or stable numbers in recent atlas projects, with all figures uncorrected for the likely greater effort in recent years and thus probably conservative. Specifically, Ohio reported stable numbers since 1965, Michigan reported no appreciable change in 20 years (from 20 to 19 squares), and New York reported a 9% drop in the number of squares (Monfils 2003; Woodliffe 2007; McGowan and Corwin 2008). In Maine, revisits to 38 known breeding sites between 1989-90 and 2005 showed a 37% decline in occupancy (Hayden et al. 2005).

In comparison, Marsh Monitoring Program results for the Great Lakes Basin, from 1995 to 2006, show an average annual decline of 10% per year, with a 95% confidence interval of -13 to -8% (Figure 4); trend and confidence intervals based on Poisson regression, corrected for overdispersion). Analysis of all of the Ontario data (i.e., not just the Great Lakes Basin) yields virtually the same trend: an annual decline of 10%, with a confidence interval of -16 to -5% (T.L. Crewe pers. comm.2008). Trend data for Canada are only available from Ontario, the only province to have started surveys early enough to provide trend estimates. These results should be viewed cautiously, however, because potential biases (see “Search effort,” above) have not been fully explored.

Figure 4. Annual abundance index of Least Bitterns detected by the Marsh Monitoring Program in the Great Lakes Basin (including both Canadian and U.S. data) from 1995 to 2006 (from Anonymous 2008). Index is Poisson mean birds per route (n = 51 routes), relative to 2006 mean (T.L. Crewe pers. comm.2008).

Chart showing the annual abundance index of Least Bitterns detected by the Marsh Monitoring Program in the Great Lakes Basin from 1995 to 2006.

In summary, historical population declines are poorly documented, but judging from rates of habitat loss (see “Habitat trends,” above), might have exceeded 80%. Trend estimates are variable according to survey program. The Ontario breeding bird atlas project suggests a decline of 32% in the number of 10x10 km squares occupied over the past 20 years, roughly equivalent to a decline of 16% in squares occupied over the past 10 years. Declines at the atlas square sampling unit are likely to underestimate the magnitude of change in abundance for Least Bittern, because some formerly occupied squares would have supported more than one pair. The Marsh Monitoring Program, which measures abundance more directly (subject to caveats given above) suggests a decline of 10% per year (confidence interval -16 to -5% per year) from 1996-2006, equivalent to a decadal decline of 65%, with a confidence interval of -83 to -40%. Thus, the best available information suggests a 10-year decline ranging between 16-65%, albeit with only limited certainty, and limited to Ontario, where the bulk of the Canadian population occurs.

Rescue effect

The Canadian range is continuous with the larger breeding range in the United States, and, like other herons, Least Bitterns are highly dispersive, with juveniles wandering widely at the end of the breeding season (see “Dispersal/Migration” above), and adults apparently shifting breeding locations across years (see “Fluctuations and trends” above). Nonetheless, given that adjacent populations in the northern United States are small and declining and frequently state-listed as threatened or endangered (see above and Table 1), the potential for a rescue effect is probably weak.

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Limiting factors and threats

Habitat loss and degradation, reviewed above under “Habitat trends”, is thought to be the most severe threat to Least Bitterns (Sandilands and Campbell 1988; Gibbs et al. 1992; James 1999), and can often be directly linked to local and regional declines in abundance and distribution (see “Population trends,” above). Most habitat loss and degradation has been anthropogenic. Historically, it consisted mainly of draining, filling, and dyking on a large scale, especially in Ontario and Quebec. More recently, even with protective measures in place for wetlands, it continues through impacts that are more local but still widespread, succinctly listed by Ingram et al.(2007) for the Great Lakes region, where they are the most severe, as: “filling, dredging and draining for conversion to other uses such as urban, agricultural, marina, and cottage development; shoreline modification; water level regulation; sediment and nutrient loading from watersheds; adjacent land use; invasive species, particularly non-native species; and climate variability and change.” Some of these impacts, notably channelization, water extraction, and erosion, can degrade Least Bittern breeding sites even when they occur well away from those sites, because they shift water levels or quality (through siltation or eutrophication) outside the narrow, stable range tolerated by Least Bitterns (Sandilands and Campbell 1988; Gibbs et al. 1992; James 1999; Weller 1999). Similar factors affect habitat on the wintering grounds, especially along the Gulf Coast and Central America (Davidson and Gauthier 1993).

Other threats are closely related to habitat loss and degradation, but may have sufficiently specific effects on Least Bitterns that they are worth singling out. These other threats include toxins, invasive species, disease, collisions, recreational activities, and climate change.

Wetlands are sinks for toxins from industrial and agricultural run-off, which predators such as Least Bitterns bioaccumulate (Eddleman et al. 1988). High dieldrin levels have been found in Least Bittern eggs in Louisiana (Causey and Graves 1969) and feathers in Ontario (Sandilands and Campbell 1988; Sandilands 2005). Herons are particularly susceptible to accumulating residues and eggshell thinning from DDE and dieldrin (Fleming et al. 1983), and Great Blue Herons (Ardea herodias) along the Great Lakes and St. Lawrence River have particularly high levels of mercury and PCBs (Champoux et al. 2006).

At sites across North America, including Ontario and Quebec, several invasive species are outcompeting the cattails in which most Least Bitterns breed. These species include Purple Loosestrife (Lythrum salicaria), Reed Canary Grass (Phalaris arundinacea), Common Reed (Phragmites australis), and, especially in Quebec, Flowering Rush (Butomus umbellatus; Lavoie et al. 2003; Hudon 2004; Jobin and Robillard 2005; Jobin 2006). All these species, as well as others that do not directly compete with cattails, such as European Frog-bit (Hydrocharis morsus-ranae), also encourage succession of marshes to drier habitat (Blossey et al.2001).

Invasive animal species are a growing concern in the Great Lakes because of their disruptive effects on ecosystem function. Direct links to Least Bitterns are undocumented but likely. Specifically, Common Carp (Cyprinus carpio) forage in ways that remove vegetation and muddy the waters so the birds cannot see their prey.

Least Bitterns 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 Eustrongilides, associated with nutrient and silt run-off, and avian salmonella, perhaps associated with sewage effluent (P. Frederick pers. comm. cited in Gibbs and Melvin 1992; Wires and Lewis 2005).

The few reports of Least Bitterns colliding with human-made structures are sufficiently dramatic to suggest the threat is locally serious. These include 16 birds killed by cars or colliding with fences in one weekend at one Louisiana refuge, regular collisions with a generator tower in Ontario that included eight on a single weekend, 18 strikes by airboats at one site in Florida, and nine road kills in one year along a 3.2 km section of highway in Florida (Sandilands and Campbell 1988; Gibbs et al. 1992; Smith and Dodd 2003).

Disturbance from recreational activities, notably boating, has been identified as a conservation concern for herons, because it can disrupt activities such as foraging and can cause nest abandonment (Kushlan and Hancock 2005). In Least Bitterns, nest abandonment has not been directly linked to human disturbance, and foraging birds reportedly habituate to frequent boat traffic (Gibbs et al. 1992). Nonetheless, this species’ first response to approach may be to freeze (Frederick et al. 1990), so without targeted studies it is hard to determine whether boat traffic disrupts foraging. Motorized boating creates waves that could flood nests and erode the marsh edges where the species forages (McConnell 2004).

Climate change is projected to lower water levels in the Great Lakes and St. Lawrence River, reducing the size and distribution of their wetlands, without compensatory gain of new wetlands (Wires and Lewis 2005; Mortsch et al. 2006). Abundance of breeding Least Bitterns along the Great Lakes has been shown to be very tightly linked to changes in water levels (Craigie et al. 2003), so if these projections hold, their abundance in southern Canada will almost certainly decline (DesGranges et al. 2006).

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Special significance of the species

The Least Bittern is not a game species and has no direct economical use or special cultural significance. Nonetheless, it has the distinction of being the smallest member of the heron family, an especially charismatic and well known taxon. Its secretive habits and attractive plumage have made it a special favourite of birdwatchers, challenging to find and emblematic of large, impenetrable wetlands. Its sensitivity to hydrology, water quality, and vegetation make it a particularly useful indicator of the health of marsh habitat in Canada (DesGranges et al.2006).

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Existing protectino or other status designations

Least Bitterns and their nests are protected under the Migratory Birds Convention Act. In Canada, the species’ listing as Threatened under Schedule 1 of the Species at Risk Act protects individuals, their residences (in this case, nest sites), and ultimately critical habitat from harm or destruction. The species is also provincially listed in Ontario as Threatened, which protects individuals, and will eventually protect their habitat, under Ontario’s Endangered Species Act, 2007. In Quebec, the species is provincially listed as Likely to be Designated as Threatened or Vulnerable, where designation as Threatened or Vulnerable offers some protection of individuals through the Act Respecting Threatened or Vulnerable Species.

The Least Bittern’s General Status Rank in Manitoba, Ontario, and New Brunswick is At Risk, and in Quebec is May Be at Risk (Canadian Endangered Species Conservation Council 2006). In Quebec, it is presently considered “Likely to be designated as threatened or vulnerable” under the Loi sur les espèces menaces ou vulnérables du Québec (Quebec’s Act Respecting Threatened or Vulnerable Species, Quebec Department of Natural Resources and Wildlife 2008). The remaining provinces that have a General Status Rank for the species rank it as Accidental (British Columbia, Saskatchewan, Nova Scotia, Prince Edward Island, and Newfoundland and Labrador).

In the United States, the species is not federally listed, but is state listed in 16 states, most of which are in the northeast and north central regions bordering Canada (Table 1). Globally, the IUCN lists Least Bitterns as Least Concern (i.e., a species that does not meet any risk category), and is thus classed together with taxa that are widespread and abundant (NatureServe 2008).

NatureServe rankings vary regionally (NatureServe 2008; Table 1). In Canada, it is ranked as N3B nationally, with provincial ranks as S2S3B in Manitoba, S3B in Ontario, S3 in Quebec, S1S2B in New Brunswick, and S1B in Nova Scotia. In the United States, it is ranked N5B, N5N, and four Gulf States give it a ranking of S4 (Florida, Georgia, and Texas) or S5 (Louisiana). Otherwise, rankings vary from S1 to S3, with no clear regional pattern (Table 1). Specifically, it is ranked as low as S1 in nine states, S2 in 18, S3 in seven, and undetermined in four.

Table 1. Conservation status of Least Bittern in the United States. Status from NatureServe 2008, listings from a web search of state wildlife departments, May 2008.
StateStatusState listing
CaliforniaS1Species of special concern
District of ColumbiaS1B,S2N 
MarylandS2S3BIn need of conservation
NevadaS2BSpecial concern
New HampshireS1 
New JerseyS3B 
New MexicoS3B,S3N 
New YorkS3B,S1NThreatened
North CarolinaS3B 
North DakotaSNRB 
Rhode IslandS2B,S2NSpecial concern
South CarolinaSNRB,SNRN 
South DakotaS2B 
TennesseeS2BDeemed in need of management
VermontS2B,S2NSpecial concern
West VirginiaS1B 

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Technical summary

Ixobrychus exilis

Least Bittern – Petit Blongios

Range of Occurrence in Canada:

Manitoba (MB), Ontario (ON), Quebec (QC), New Brunswick (NB), and Nova Scotia (NS)

Demographic Information

Generation time (average age of parents in the population)
< 3 yrs
Estimated percent reduction in total number of mature individuals over the last 10 years or 3 generations.
Based on 65% population decline estimated from Marsh Monitoring Program around Great Lakes (biased because this is the region where the strongest declines have likely occurred) and 16% decline in occurrence estimated from Ontario Breeding Bird Atlas (biased under-estimate of population decline). No trend information is currently available outside Ontario, but the majority of the Canadian population occurs in southern Ontario.>30%
Suspected percent reduction in total number of mature individuals over the next 10 years or 3 generations.
Unknown but likely >10%
Estimated percent reduction in total number of mature individuals over any 10 years or 3 generations period, over a time period including both the past and the future.
Are the causes of the decline clearly reversible?
Are the causes of the decline understood?
Not fully
Have the causes of the decline ceased?
[Observed, inferred, or projected] trend in number of populations:
Not applicable
Are there extreme fluctuations in number of mature individuals?
Are there extreme fluctuations in number of populations?
Not applicable

Extent and Area Information

Estimated extent of occurrence - Measured as a minimum convex polygon
1,331,000 km²
Observed trend in extent of occurrence
Are there extreme fluctuations in extent of occurrence?
Index of area of occupancy (IAO)
IAO cannot be calculated at this time because of large uncertainties in distribution and abundance: Unknown km²
Inferred trend in area of occupancy:
Are there extreme fluctuations in area of occupancy?
Is the total population severely fragmented?
Number of current locations:
Trend in number of locations:
Are there extreme fluctuations in number of locations?
Trend in area and quality of habitat:

Number of mature individuals in each population

Total (based on Ontario Breeding Bird Atlas and surveys outside Ontario):
Ca. 3000 individuals
Number of populations (locations):
1 population

Quantitative Analysis

Not done

Threats (actual or imminent, to populations or habitats)

Loss and degradation of wetland habitats, toxins, invasive species, disease, collisions, and climate change.

Rescue Effect (immigration from an outside source)

Status of outside population(s)?
USA: small and declining in northeastern and northcentral states bordering Canada; stable elsewhere
Is immigration known?
Unknown, but possible
Would immigrants be adapted to survive in Canada?
Is there sufficient habitat for immigrants in Canada?: Declining
Is rescue from outside populations likely?
Not likely

Current Status

  • COSEWIC: Threatened (2009, 2001)
  • Ontario: Threatened
  • Quebec: Likely to be Designated as Threatened or Vulnerable

Additional Sources of Information

Not applicable.

Status and Reasons for Designation

Status: Threatened

Alpha-numeric code: A2b; C1

Reasons for designation: This diminutive member of the heron family has a preference for nesting near pools of open water in relatively large marshes that are dominated by cattail and other robust emergent plants. Its breeding range extends from southeastern Canada through much of the eastern U.S. Information on the population size and exact distribution of this secretive species is somewhat limited. Nevertheless, the best available evidence indicates that the population is small (about 3000 individuals) and declining (>30% in the last 10 years), largely owing to the loss and degradation of high-quality marsh habitats across its range.

Applicability of Criteria

  • Criterion A (Decline in Total Number of Mature Individuals): Meets Threatened A2b, with a reduction in population size of >30% in the last three generations, based upon appropriate indices of abundance.
  • Criterion B (Small Distribution Range and Decline or Fluctuation): Not applicable.
  • Criterion C (Small and Declining Number of Mature Individuals): Meets Threatened C1, with estimated population of <10,000 individuals and continuing decline estimated at >10% over three generations.
  • Criterion D (Very Small Population or Restricted Distribution): Not applicable
  • Criterion E (Quantitative Analysis): Not done.

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Acknowledgements and Authorities consulted


Members of the Least Bittern Recovery Team provided data that were essential for preparing this report, especially Ron Bazin, Benoît Jobin, Laurie Maynard, and Jen Stewart. Angela McConnell laid the groundwork for this report with her exhaustive literature review for the recovery team. All the authorities listed below found time in busy schedules to provide useful advice, perspectives, and, in some cases, data from their region. The report also benefited from comments provided by Theresa Aniskowicz, Ron Bazin, Peter Blancher, Mark Brigham, Alan Dextrase, Rhonda Donley, Christian Friis, Benoît Jobin, Burke Korol, Shawn Meyer, Barb Slezak, Don Sutherland, and Peter Thomas. Thanks go to Bird Studies Canada for supplying Marsh Monitoring Program data, and to all the volunteer participants who gathered data for that project. Alain Filion of the COSEWIC Secretariat helped calculate the extent of occurrence and area of occupancy. Thanks to Benoît Jobin (Canadian Wildlife Service) for permission to use his cover photograph.

Authorities consulted

  • Banville, Daniel. Biologist, Coordonnateur provincial aux espèces fauniques menacées ou vulnérables, Ministère des Ressources naturelles et de la Faune, Sainte-Foy, QC.
  • Bazin, Ron. Wildlife Biologist, Species at Risk Recovery Unit, Environment Canada, Winnipeg, MB.
  • Blaney, Sean. Botanist & Assistant Director, Atlantic Canada Conservation Data Centre, Sackville, NB.
  • Crewe, Tara L. Bird Conservation Biologist, Bird Studies Canada, Port Rowan, ON.
  • De Smet, Ken. Species at Risk Biologist, Manitoba Conservation Data Centre, Winnipeg, MB.
  • Doucet, Denis. Zoologist, Atlantic Canada Conservation Data Centre, Sackville, NB.
  • Elderkin, Mark F. Species at Risk Biologist, Department of Natural Resources, Government of Nova Scotia, Kentville, NS.
  • Filion, Alain. Scientific and Geomatics Project Officer, COSEWIC Secretariat, Environment Canada, Ottawa, ON.
  • Goulet, Gloria. Coordinator - Aboriginal Traditional Knowledge, COSEWIC Secretariat, Environment Canada, Ottawa, ON.
  • Jobin, Benoît. Species at Risk Biologist, Ecosystem Conservation, Environment Canada, Saint-Foy, QC.
  • Jones, Kathy. Aquatic Surveys Volunteer and Data Coordinator, Bird Studies Canada, Port Rowan, ON.
  • Maynard, Laurie. Ecosystem Conservation, Environment Canada, Downsview, ON.
  • Nantel, Patrick. Conservation Biologist, Species at Risk Program, Parks Canada, Gatineau, QC.
  • Stewart, Jen. Wildlife Biologist – Species at Risk Recovery, Canadian Wildlife Service, Environment Canada, Sackville, NB.
  • Sutherland, Don. Zoologist, Ontario Natural Heritage Information Centre, Ontario Ministry of Natural Resources, Peterborough, ON.
  • Toner, Maureen. Biologist, Species at Risk Program, Fish and Wildlife Branch, Department of Natural Resources, Fredericton, NB.

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Biographical summary of report writer

Andrew Gregg Horn grew up in Cambridge, Massachusetts, earned his B. Sc. at Cornell University in 1981, and did his Ph.D in Zoology at University of Toronto on song complexity in Western Meadowlarks Sturnella neglecta. During post-doctoral work at the University of Cambridge, Queen’s University, and Agriculture Canada, he studied various aspects of parent-offspring interactions and acoustic communication in birds, work he continues as a Research Adjunct at Dalhousie University. He has also conducted various projects in avian monitoring and assessment, drafting several status reports and recovery plans, which include the first draft of the Canadian Least Bittern Recovery Strategy.

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