Canada Warbler

The Canada Warbler is a bird of moist forests and swamps over most of its range and can be found in both coniferous and deciduous habitats. Key microhabitat features typically include a dense understory and mossy ground cover. Even though breeding habitat often has standing water, the nest is usually built on the ground, albeit on a hummock, upturned root mass, or other slightly raised area. It is also usually concealed by some sort of overhang, including living vegetation, fallen slicks or logs, or dead leaves.

A female will lay 4-5 eggs, but research on paternity in this species has shown that these are not necessarily all from the same male. In a study in New Hampshire, roughly half the young raised had a male parent different from the one mated to the nesting female. Both males and females will seek out mating opportunities with birds other than their immediate mate, presumably to diversify the genetics of their offspring. Such fooling around is not atypical in songbirds that we typically think of as forming a durable pair bond over the course of a breeding season.

Like many of our long-distance migrants, Canada Warblers are declining, although in some parts of their range to the north this decline may have slowed down. Habitat loss has often been implicated in the decline, including forest fragmentation and loss of understory shrubs (e.g., due to over-browsing by deer). Forest maturation may be another factor, as such forests often have less developed shrub layers and some forms of forest thinning (including severe storms) create conditions where this layer can quickly regenerate. There is also concern about habitat loss in the Canada Warbler’s winter range in the northern Andes. Although it frequently uses disturbed habitats in parts of South America, even these are at risk as development and agriculture expand with increasing human populations.