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COSEWIC Assessment and Update Status Report on the Cerulean Warbler in Canada

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COSEWIC
Executive Summary

Cerulean Warbler
Dendroica Cerulea

Species Information

The Cerulean Warbler (Dendroica cerulea Wilson, Paruline azurée) is a small (8-10 g) wood-warbler.

 

Distribution

This species breeds in the deciduous forests of eastern North America but has a very patchy distribution. The Cerulean Warbler winters in the Andes Mountains of South America, from Venezuela to Bolivia. The Canadian breeding range consists of two main geographic clusters in Ontario and a small number of breeding individuals southwestern Quebec.

 

Habitat

On the breeding grounds, Cerulean Warblers are associated with mature deciduous forest with large and tall trees and an open understory. They are found in both wet bottomland forests and upland locations on dry ridges. In Ontario, they also occupy older second-growth deciduous forests. At smaller spatial scales, Cerulean Warblers exhibit strong preferences for certain microhabitats. Canopy configuration (e.g. foliage stratification, gap distribution, tree species distribution) may be the most important predictor of breeding habitat suitability for Cerulean Warblers.

In eastern Ontario, 70-80% of the original deciduous forest had been removed by the 1880s. However, over the last century, there has been substantial re-growth of forests in eastern Ontario that parallels what has been observed in the northeastern United States over the last several decades. Cerulean Warblers winter in mature, humid evergreen forests in South America, which are also ideal for human settlement and agriculture, notably the production of coffee. As a consequence, large tracts of winter habitat have been drastically altered.  Fortunately, Cerulean Warblers can use modified forests (e.g. shade-coffee plantations) as winter habitat.

 

Biology

Despite broad interest in the Cerulean Warbler, the basic biology of this species remains poorly documented and understood. Breeding pairs generally raise only a single brood per year, although double brooding has been recorded. Over an 8-year period in eastern Ontario, average fecundity was 1.9 fledglings per breeding pair although there were large annual fluctuations. In a genetic study of five Cerulean Warbler geographic clusters throughout the breeding range (including 2 in Ontario), estimates from microsatellite data revealed sufficient levels of gene flow to prevent genetic differentiation through drift. Thus, dispersal between geographic clusters (presumably by young birds) undoubtedly plays an important role in Cerulean Warbler population dynamics.

Individuals appear to be strictly insectivorous during the breeding season but will avail themselves of nectar during the non-breeding season. Nestlings and fledglings are fed larval lepidopterans almost exclusively.

The species appears to be relatively resilient in the face of habitat disturbance, both anthropogenic and natural. In eastern Ontario, it breeds successfully in forests managed for the production of maple syrup and for shelterwood silviculture. Cerulean Warblers have also exhibited resilience to habitat damage resulting from the January 1998 ice storm.

Cerulean Warblers exhibit two behaviours that may render them particularly vulnerable. The first is the high site fidelity exhibited by adults. Despite their apparent resilience to certain disturbances, the fact that adults seem unable to “recognize” habitat degradation may keep birds in unsuitable habitats. The second is their protracted migration periods in both spring (2 months) and fall (4 months). This long traveling period could be subjecting individuals to high physiological stresses and high probability of predation, as well as habitat disturbances along the length of their migration routes.

 

Population Sizes and Trends

The Cerulean Warbler exhibited the greatest decline in abundance for any species of North American wood-warbler for the period 1966-2000. However, it is thought to be expanding its range in the northeastern United States and in southern Ontario and Quebec. It is unclear if the apparent increase in Canadian Cerulean Warbler populations over the last 50 years represents a true range expansion into new areas, a re-colonization event, or an artifact of the increase in public and scientific interest.

The Canadian population is estimated to be 500 - 1000 breeding pairs. Unfortunately, given the patchiness of its habitat and the difficulties associated with surveying this species, it is difficult to predict where the high end of a population estimate would fall. Current best estimates of the size of the North American population range between 85 000 - 287 000 pairs.

 

Limiting Factors and Threats

The three dominant limiting factors for this species are: habitat destruction on breeding, migration, and wintering grounds; fragmentation of existing habitats; and environmental degradation (e.g. acid rain).

 

Special Significance of the Species

The Cerulean Warbler has generated considerable public, scientific, and conservation interest. The positive regard in which this species is held has led to an intense public interest in its conservation. While it does not appear to fulfill any critical ecological role, it has become a symbol of the health of mature deciduous forests in eastern North America. In Ontario, management of forested habitats for Cerulean Warblers will likely have a positive effect on other forest-interior species of interest.

 

Existing Protection or Other Status Designations

The Cerulean Warbler is not federally listed in the United States and is considered a Species of Special Concern in Canada by COSEWIC. The Migratory Bird Treaty Act of 1918 provides protection against direct take. NatureServe gives it a global rank of G4 (common).

 

Summary of Status Report

Since the publication of the first status report for the Cerulean Warbler in Canada, the Canadian population appears to have remained stable or possibly to have decreased, both in terms of population size and distribution.  Given what is known about Cerulean Warbler habitat requirements, the dominant limiting factors for this species will always be limiting, regardless of population size. However, the largest potential threat to the long-term health of this species is that its basic biology remains poorly documented and understood which has direct impact on our ability to manage and sustain its populations, most notably in the areas of mating and social systems, landscape-scale habitat selection, and migration stopover ecology.