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

Population Size and Trend

The Cerulean Warbler has exhibited the greatest decline in abundance for any species of North American wood-warbler -- an average decline of 3% per year based on Breeding Bird Survey data for the period 1966-2000 (Robbins et al. 1992, Link and Sauer 2002). In fact, only six species of passerine exhibited more severe declines. The greatest declines were reported at the core of the breeding range (Tennessee, Kentucky, Ohio and West Virginia) where abundances were highest (Robbins et al. 1992, James et al. 1996, Villard and Maurer 1996).

In the northeastern United States and in southern Ontario and Quebec, the Cerulean Warbler is currently thought to be expanding both its range and its abundance (Hamel 2000a, Rosenberg et al. 2000). Unfortunately, it is difficult to place the status of the current Canadian population into a historical context. Early reports of this species in Ontario are inconsistent and make it difficult to ascertain whether or not its apparent increase over the last 50 years represents an expansion into new areas (i.e., Cerulean Warblers are new to Ontario), a recolonization (i.e., Cerulean Warblers were once common in Ontario and are only now returning as forests re-grow, especially in eastern Ontario) or an artifact of the increase in the ability and desire of bird-watchers and researchers to locate this inconspicuous species.

Macoun and Macoun (1909 in McCracken 1993) classified the Cerulean Warbler as common in southwestern Ontario at the turn of the last century. It is not considered a common bird in the same area today. Eagles (1987) classified it as an uncommon local breeding bird in southern Ontario. This possible change in abundance is likely due to the continued historical loss of breeding habitat in the region (Eagles 1987). The historical status of the Cerulean Warbler in eastern Ontario is even less clear. In the 1920s, DeLury (1922) felt that the Cerulean Warbler was more common in eastern Ontario (even as far north as the Ottawa Valley) than people assumed. Broley (1929) believed that Cerulean Warblers bred north of Kingston as early as 1929 although the first documented nest in the Kingston region was found in the early 1950s (Quilliam 1965). 

The database of the Ontario Natural Heritage Information Centre (NHIC) lists 45 Element Occurrences in Ontario; most of these occurrences represent small numbers of individuals. The largest geographical cluster in southwestern Ontario only supports 20-30 breeding pairs annually. In fact, there are probably fewer than 100 breeding pairs of Cerulean Warblers in southwestern Ontario (NHIC data). The single largest geographical cluster (~250 pairs) in Canada is currently on the property of the Queen’s University Biological Station north of Kingston, ON (Jones 2000; Jones et al. 2000a).

The initial results of the current Ontario breeding bird atlas project (2001-2 data) imply that the distribution of Cerulean Warblers has possibly contracted since the first atlas of 1981-5 (Figure 4). For example, in the Perth Region, 7 atlas squares reported breeding evidence in 1981-5; only 2 have done so in the current atlas effort. Similarly, in the 1000 Islands, 5 squares reported breeding evidence in the first atlas versus 0 in the current effort. Once the current atlas effort is completed, it will be possible to make a more accurate determination of a population trend. 

The only long-term demographic data set (1994 to present) on Cerulean Warblers in Ontario indicates that the local “population” at the Queen’s University Biological Station has remained relatively constant over the last 8 years (Jones et al. in review). However, this persistence is likely only due to birds immigrating from other areas as it is not producing enough fledglings to sustain itself (Jones et al. in review); the reasons why not are unclear at this time.

Most of the land that comprises the range of Cerulean Warblers in Ontario is privately owned. However, the species is found in some protected areas including national parks (e.g., St. Lawrence Islands National Park, Leggo pers. com. 2002), as well as provincial parks in both Ontario (e.g., Charleston Lake Provincial Park, Jones pers. obs.) and Quebec (e.g., Parc du Mont Saint-Bruno, Shaffer pers. com. 2002). Other protected areas include forests owned and managed by the Middlesex County and forests in Haldimand/Norfolk (owned and managed by the Long Point Region Conservation Authority).

The population of breeding birds in Quebec is at least as old as the population in eastern Ontario (first confirmed record in 1950) but the number of breeding birds is very small (less than 40 pairs; BDOMQ; Shaffer pers. com. 2002).

The combination of the NHIC data with the trends exhibited by the current Ontario Breeding Bird Atlas and data from researchers at Queen’s University (Jones et al. unpublished data) lead us to estimate that the current population of Cerulean Warblers in Canada is between 500 and 1000 breeding pairs. The bulk of this population is found in eastern Ontario. 

Current estimates of the size of the North American Cerulean Warbler population range from 85 000 to 287 000 pairs (Rosenberg et al. 2000, pers. comm. 2002).