Introduction

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 (1953) 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. Spraying for spruce budworm during the 1970s and 1980s, primarily in Canada, has partially controlled these outbreaks, reducing 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 (Dendroica striata) and Pine Warblers (D. pinus) in Basic plumage, making it difficult to distinguish these species (Kaufman 1990). 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.