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Canada Warbler
Cardellina canadensis
NatureServe conservation status
Global (G-rank): G5
State (S-rank): SNA
External links
Species range
Breeding range extends from southeastern Yukon, northeastern British Columbia, and northern Alberta across southern Canada to Nova Scotia, and south to southern Manitoba, northern Minnesota, Great Lakes region, southern Appalachians (to northern Georgia), northern New Jersey, southeastern New York, and southern New England (AOU 1998, Conway 1999). Range during the boreal winter extends from Venezuela and northern Colombia south to Ecuador, southern Peru, and the Tepui region of northern Brazil, mostly in and east of the Andes, and rarely northward to Central America (AOU 1998, Conway 1999). Birdlife International (2014) estimates a range of 2.1 million square kilometers.
Migration
Migration through Central America and eastern Mexico occurs in both fall and spring. This species is a common migrant in Costa Rica in early September-early November; uncommon transient in April-early May (Stiles and Skutch 1989). It is present in South America mostly in October-April (Ridgely and Tudor 1989). On Appledore Island, Maine, individuals have significantly longer stopovers in fall than in spring (Morris et al. 1994).
Habitat
Breeding habitat includes moist thickets of woodland undergrowth (especially aspen-poplar), bogs, tall shrubbery along streams or near swamps, and deciduous second growth. Habitat is more specific in localized regions. For example, the species is limited to forested wetlands in Rhode Island (Miller 1999) and hemlock-dominated ravines in Ohio (Mitchell 1999). Habitat predictors in western Maryland included limited ground cover but high foliage density between 0.3 and 1 meter (Robbins et al. 1989). Habitat in Rhode Island also included limited ground cover (a negative correlation with deciduous foliage cover within 0.5 meter of the ground) and a thick shrub layer (a positive correlation with foliage cover between 2 and 4 meters; Mitchell 1999). In northeastern British Columbia this warbler is associated with wet, usually unstable slopes in deciduous or mixed forests with a well-developed shrub layer and considerable amounts of woody debris (Campbell et al. 2001). Nests are on or near the ground, among roots of fallen trees, in cavities in banks, or on ledges, sides of rocks, hummocks, stumps, or fallen logs, or on the ground under shrubbery (Harrison 1978). In migration, this warbler uses various forest, woodland, scrub, and thicket habitats, mostly in humid areas. In winter, it occurs in forested areas of foothills and mountains.
Food habits
Diet includes beetles, mosquitoes, flies, moths, smooth caterpillars, etc.
Ecology
This species appears to show positive numerical response to outbreaks of the spruce budworm (Crawford and Jennings 1989, Patten and Burger 1998), though it is not generally considered to be a "budworm specialist."
Reproductive characteristics
Eggs are laid in May-June. Clutch size is three to five (usually four). Young are tended by both parents (Terres 1980, Harrison 1978).
Threats or limiting factors
Declines evident in Breeding Bird Survey data may be related to change in forest structure over the past century, combined with loss of forested wetlands (Conway 1999). Forest regeneration of previously farmed lands in the Northeast probably provided optimal habitat (forest with dense understory) in the early and mid-1900s, but continued forest maturation probably eliminated the understory, and habitat became less suitable for the Canada Warbler (Conway 1999). Additionally, a large area of forested wetlands in the Northeast were drained, filled, and developed between 1950 and 1980 (Tiner 1984). Infestations of the hemlock woolly adelgid have degraded or eliminated habitat in some areas. Miller (1999) reported that declines in Rhode Island may be related to the impingement of urban development on heavily forested landscapes. This species appears to be sensitive to forest fragmentation; density and probability of occurrence in a forest decline with forest area (reviewed by Conway 1999).Loss of wintering habitat in the northern Andes region of South America may have contributed to the decline (Robinson 1997, Faccio et al. 1997). Diamond (1991) projected that from 1985 to 2000, 28 percent of wintering habitat would be lost. Declines in undisturbed breeding habitats in Vermont (Faccio et al. 1997) suggest the problem is more than loss of breeding habitat, unless there is an undiscovered link between the advancement of succession on those monitoring sites and the reported declines.Regardless of location, habitat loss is a primary concern. No other major threats have been identified.
