COSEWIC Assessment and Update Status Report on the Cerulean Warbler in Canada
- Assessment Summary
- Executive Summary
- COSEWIC Mandate, Membership and Definitions
- Lists of Figures and Tables
- Species Information
- Population Size and Trend
- Limiting Factors and Threats
- Special Significance of the Species
- Existing Protection or Other Status
- Summary of Status Report
- Technical Summary
- Acknowledgements, Literature Cited, and Biographical Summary of Contractors
- Authorities Consulted and Collections Examined
a. Breeding grounds -- Cerulean Warblers are traditionally associated with forested landscapes, particularly mature deciduous forest with large and tall trees and an open understory (Hamel 2000a). They are found in both wet bottomland forests as well as upland locations on mesic slopes. In Ontario, they also occupy older second-growth deciduous forests (Peck and James 1987; Jones and Robertson 2001). The Cerulean Warbler is usually considered an area-sensitive species (Robbins et al. 1992; Hamel 2000a). Minimum area requirements reported in the literature range from 20‑30 ha in Ohio to 1600 ha in Tennessee (Robbins et al. 1992; Hamel 2000a). In eastern Ontario, Cerulean Warblers have been found breeding successfully in forest fragments as small as 10 ha (Jones and Robertson unpublished data). The spatial distribution of forest patches (especially distance between suitable patches) undoubtedly plays an important role in settlement patterns and area sensitivity. This needs further study.
At smaller spatial scales, Cerulean Warblers exhibit strong preferences for certain microhabitats. Territories are usually characterized by well-spaced large trees, with high canopies and dense foliage cover in the upper midstory and canopy, and birds tend to avoid areas of dense understory (Hamel 2000a; Jones and Robertson 2001). Individuals spend the majority of their time in the upper reaches of the canopy. There appears to be no consistent tree species preference for nest locations (Oliarnyk and Robertson 1996; Hamel 2000a; Jones and Robertson 2001). Several researchers have reported on the apparent importance of internal canopy gaps as a component of successful Cerulean Warbler territories (Bent 1953; Harrison 1984; Oliarnyk and Robertson 1996).
In eastern Ontario, Cerulean Warblers do not use all parts of their territories equally (Barg 2002). Male Cerulean Warblers have distinct areas of high use (hereafter referred to as core areas) that are associated with concentrated singing activity. These core areas are both vegetatively and structurally distinct from the rest of the territory. In particular, core areas are dominated by bitternut hickory (Carya cordiformis), a species that is among the last to fully leaf-out in eastern Ontario. As the Cerulean Warbler song does not appear to be particularly well suited for transmission in densely forested habitats (Woodward 1995), it is hypothesized that males are selecting these core areas to maximize song propagation (Barg 2002). It appears that canopy configuration (e.g. foliage stratification, gap distribution, tree species distribution) may be an important predictor of breeding habitat suitability for this species.
b. Migration -- Very little substantive information exists on habitat requirements during migration. Individuals have been observed in lower montane wet forest in Belize (Parker 1994), as well as in primary and secondary forests in Guatemala (Land 1970), Costa Rica (Stiles and Skutch 1989) and Panama (Ridgely and Gwynne 1989).
c. Winter grounds -- Cerulean Warblers winter on the eastern slope of the Andes Mountains of South America in mature, humid evergreen forests. They also use modified forests (e.g. shade-coffee plantations) as overwinter habitats (Jones et al. 2000b, 2002).
In eastern Ontario, 70-80% of the original deciduous forest had been removed by the 1880s, largely through the establishment of settlers and the logging industry (Keddy 1994; OMNR 1997). However, shallow soils rendered much of the cleared land un-arable and, over the last century, there has been a gradual shift away from agriculture as a dominant economic force in the region. As a consequence, there has been substantial re-growth of forests in eastern Ontario; the average overall forest cover in eastern Ontario is expected to level out at approximately 40% (OMNR 1997). This re-growth parallels what has been observed in certain parts of the eastern United States over the last several decades (Askins 1993). However, without detailed assessments of site fidelity and reproductive success it is difficult to determine whether this apparent increase in habitat availability is paralleled by a simultaneous increase in habitat quality. Furthermore, this apparent increase in habitat availability is highly regionalized. In southwestern Ontario, mature growth stands are becoming increasingly rare, in part due to a lack of involvement by the Ontario Ministry of Natural Resources in the management of private woodlots (Friesen pers. comm. 2002).
On the wintering grounds in South America, the preferred habitats of Cerulean Warblers occur in a landscape that is also ideal for human settlement and agriculture, notably the production of coffee, cacao, tea, hill rice, and coca (Robbins et al. 1992; Stotz et al. 1996). Other than the Atlantic forests of Brazil, the humid montane forests of South America have been altered more drastically than any other South American forest type (Robbins et al. 1992; Stotz et al. 1996).
The bulk of Cerulean Warbler habitat in Ontario is privately owned although relatively large numbers are found on public protected lands (Jones pers. obs.). At this point, more precise estimates on the numbers of birds breeding on these lands are unavailable. Based on preliminary evidence (Jones unpubl. data), we estimate that 10‑20% of the Canadian population is found on public protected lands but further surveys are needed to increase the precision and accuracy of this estimate.
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