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This little-studied New World warbler breeds in boreal Canada and the northeastern United States and Appalachian Mountains south to Tennessee and Georgia. When breeding, it is most abundant in cool, moist forests with a mix of coniferous and deciduous trees, a dense understory with subcanopy trees that are used as song perches, and complex ground cover. It frequents rhododendron thickets in Appalachian forests, aspen (Populus) forests in western boreal regions, and forested wetlands in the central part of its breeding range and on the margins of the Maritimes. It often occurs in areas having abundant moss cover and poor drainage, nesting on or near the ground in recessed pockets within moss hummocks, upturned tree-root masses, under woody debris, or within small hillocks with deep litter and dense saplings. In boreal and hemiboreal landscapes of Quebec and Ontario, it also occupies stands dominated by trembling aspen (P. tremuloides), white birch (Betula papyrifera), and balsam fir (Abies balsamea) with an understory of mountain maple (Acer spicatum), white birch, and beaked hazelnut (Corylys cornuta) saplings (S. Dufour-Pelletier, unpublished data). The Canada Warbler spends little time on the breeding grounds, being one of the last warbler species to arrive in spring and among the first to depart after breeding. This species undertakes a long-distance migration between its breeding grounds and overwintering areas in northern South America, a long migration for a New World warbler.
To obtain insect and spider prey, Canada Warblers use a variety of foraging techniques, but primarily foliage gleaning and flycatching. It is an active forager, its tail often cocked and wings flicking. It frequently uses aerial maneuvers, which explains historical names used for the species: Canadian Flycatcher and Canadian Flycatching Warbler. It is socially monogamous and territorial during the breeding season and often joins mixed-species foraging flocks during nonbreeding periods. Some individuals are territorial and join flocks opportunistically, whereas others appear to be stable members of such flocks.
Recent studies of nesting populations have yielded new information on breeding biology. Nest site fidelity (and often mate fidelity) are high, and some individuals have bred in local populations up to 9 years. Females lay 1 egg per day and have a short incubation period, lasting just 10 days after the final egg is laid. The nestling period ranges from 7 to 9 days.
Populations have declined steadily over the past 50 years, likely in response to forest succession and loss of forested wetlands on the breeding grounds, making the species a high priority for monitoring and management. The Canada Warbler is classified as threatened in Canada and a species of conservation concern in many states in the United States. Its overwintering grounds along the east slope of the Andes are also under threat, though the species will use disturbed forests if sufficient forest cover remains. Still, the 62% overall decline in breeding populations since 1966 (1) underscores the need for active research and conservation efforts.