Prothonotary warbler (Protonotaria citrea) COSEWIC assessment and status report: chapter 2

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Executive Summary

Species information

The Prothonotary Warbler is one of North America’s most dazzling songbirds. Males and females look alike, but males are more brightly coloured. Both have golden yellow heads and breasts, olive-green backs, and blue-grey wings and tails. White tail spots are quite prominent. Prothonotary Warblers are small birds, weighing about 14 grams, and measuring about 14 cm long. 

Distribution

The Prothonotary Warbler breeds throughout much of the eastern U.S. north as far as southwestern Ontario. It is most abundant in the southeastern U.S. Populations in Canada are small and scattered, and are found mostly along the north shore of Lake Erie. Its wintering range extends from the coastal lowlands of southern Mexico, south to coastal areas of northern South America.

Habitat

During the breeding season, Prothonotary Warblers occupy large, mature and semi-mature, deciduous swamp forest and riparian floodplains. Permanent and semi-permanent pools of open water are characteristic, and nests are typically situated over standing or slow-moving water. The species nests in natural cavities and those excavated by other species, using small, shallow cavities that are situated at low heights. Properly designed nest boxes are also readily accepted. Males often build one or more incomplete ("dummy") nests. Nesting densities are at least partially limited by the availability of suitable cavities. The Prothonotary Warbler's key wintering habitat is coastal mangrove forest in Central America and northern South America. It also winters in swamps and wet woodlands, mainly below 1300 m.

Biology

The Prothonotary Warbler is highly territorial during the breeding season, but not on the wintering grounds. Territory size (about 1-2 ha) is influenced by habitat quality, habitat configuration and population density. The typical clutch size is 4-6 eggs. In the southern U.S., the species is usually double-brooded, while populations in the north (including Canada) are typically single-brooded. Nesting success is highly variable, and largely depends on predation rates. Destruction of eggs and young by House Wrens is a major source of nest failure at sites where wrens are common. For a cavity-nesting species, the Prothonotary Warbler shows an exceptionally high level of cowbird parasitism (27% in Ontario). In Ontario, annual nest success over a seven-year period ranged from 44% to 67%.

The average life span of males is about 2.5 years. Probability estimates for annual survival of adults are about 53% for males and 47% for females. In Canada, an average of about 37% of all territorial males remain unmated during the course of the breeding season. An apparent shortage of females in the Canadian population limits reproductive potential. The Canadian population is also strongly skewed in favour of older birds, indicating poor recruitment/immigration.

Prothonotary Warblers are insectivores, feeding mostly on caterpillars, flies, midges, and spiders.

Population sizes and trends

The continental population of Prothonotary Warbler is estimated at about 900,000 pairs, over 99% of which reside in the U.S., mostly in the southeastern states. In recent years, Canada has supported no more than 20 pairs, plus several unmated territorial males. The current (i.e. 2005) population estimate in Canada is between 28 and 34 individuals, including unmated males.

Breeding Bird Survey results indicate that the continental population of Prothonotary Warbler has experienced a significant long-term decline, averaging - 1.3%/year from 1966-2005 (40% overall). In Canada, information from intensive surveys that target Prothonotary Warblers suggest that the population has decreased from an estimated 40 pairs in 1995 to 8 pairs in 2005, which amounts to an 80% decrease in population size in the last decade.

Despite the population decline, results of the Ontario Breeding Bird Atlas indicate that the overall distribution of the species in Canada has remained relatively unchanged over the past 20 years.

Limiting factors and threats

In Canada, the range of the Prothonotary Warbler appears to be limited primarily by climate and habitat availability. The following threats have been identified: range-wide loss and degradation of swamp forest breeding habitat; loss of wintering habitat especially widespread destruction of mangrove forest; habitat disturbance that increases nest site competition and reduces breeding productivity (primarily due to interspecific egg destruction by wrens and/or cowbird parasitism); invasive forest insects (e.g. emerald ash borer) that have potential to create serious habitat disturbances; invasive plants (e.g. common reed and European black alder) that render habitat unsuitable; and catastrophic weather events and changes in precipitation related to climate change.

Special significance of the species

Within its breeding range, the Prothonotary Warbler may be a useful indicator of the quality of forested wetlands. During spring migration in Canada, it has local socio-economic benefits, because of its intense popularity with bird watchers.

Existing protection

The Prothonotary Warbler is presently regulated as an Endangered species under Canada’s Species at Risk Act and by Ontario’s Endangered Species Act. General protection is also afforded through the Migratory Birds Convention Act. Additional provincial regulations and policies, which promote protection of its breeding habitat in Ontario, also benefit the species. In addition, the majority of the breeding population now remaining in Ontario occurs on public, protected lands.

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 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. 

Species

Any indigenous species, subspecies, variety, or geographically defined population of wild fauna and flora.

Extinct (X)

A species that no longer exists.

Extirpated (XT)

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

Endangered (E)

A species facing imminent extirpation or extinction.

Threatened (T)

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

Special Concern (SC)*

A species of special concern because of characteristics that make it particularly sensitive to human activities or natural events.

Not at Risk (NAR)*

A species that has been evaluated and found to be not at risk.

Data Deficient (DD)***

A species for which there is insufficient scientific information to support status designation.

* 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.

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.

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