The Bay-breasted Warbler, a neotropical migrant, inhabits boreal coniferous forests in a broad band across central and eastern Canada. This species breeds primarily in northern spruce (Picea)-fir (Abies) forests, feeding and nesting in the dense foliage of conifers, and typically undergoes significant changes in population density correlated with outbreaks of spruce budworm (Choristoneura fumiferana) and other caterpillars.
Since Bent's (Bent 1953b) summary of the life history of this species, several studies have focused on changes in its populations within its breeding range in relation to budworm outbreaks (Venier and Holmes 2010). The decline in spruce budworm populations since the early 1980's has correlated with reductions of Bay-breasted Warbler populations. During outbreak years, it was estimated that this species typically ate more than 13,000 budworms per hectare in a 41-day period (Crawford and Jennings 1989). Despite these studies within the breeding range of this species, many aspects of its breeding biology remain unknown (e.g., song patterns and repertoire, pair formation, nest selection, and egg-laying.)
The Bay-breasted Warbler winters in Panama and northern South America. An insectivore on its breeding grounds, it eats mostly fruit in Panama during the tropical dry season.
In breeding plumage, males differ markedly from females. Nonbreeding males, females, and juveniles closely resemble Blackpoll Warblers (Setophaga striata) and Pine Warblers (S. pinus) in Basic plumage, making it difficult to distinguish these species. Only since the 1950s have the migration routes of the Bay-breasted Warbler become clarified as people have learned to properly distinguish these three species of confusing fall warblers.
Recently, the Bay-breasted Warbler has been placed on the Partners in Flight Watch List (Rich et al 2004) in the category of moderately abundant or widespread but with declines or high threats. In Canada, which contains over 90% of its range, the Bay-breasted Warbler declined 3% annually from 1970 – 2009 and 5.2 % annually from 1989 to 2009 (Environment Canada 2010). These declines are significantly correlated with decreasing spruce budworm populations, as measured by area defoliated (Venier et al. in review). Although Bay-breasted Warbler has been established as an old forest specialist, it appears to expand its habitat into young and intermediate aged stands in response to budworm outbreaks.