Bay-breasted Warbler

Setophaga castanea

Chestnut flanks and a quiet song high in the spruces. Population rises and falls with the budworm.

Population: ~9.9 million mature individuals, increasing (Partners in Flight/BirdLife, 2019 data; ~24% increase over the past decade after a small historical decline 1970-2017)
Habitat: Breeds in boreal spruce-fir coniferous forest across Canada and the northeastern US; winters in wet lowland and moist tropical forest, foraging low in dense foliage
Fun fact: Bay-breasted Warbler populations boom and crash with spruce budworm outbreaks — during infestations the birds gorge on the larvae and can raise unusually large broods, making them one of the few songbirds whose numbers track an insect pest almost one-to-one.
Range: Breeds across eastern and central Canada and the northeastern United States; migrates through the Great Lakes region and Appalachians/Atlantic coast; winters in Central America, the Caribbean, and northern South America (Colombia to Venezuela) — distributed across ~28 countries/territories
Diet: Insectivorous in the breeding season, with a strong preference for spruce budworm larvae (a key population driver); shifts to a diet dominated by fruit on tropical wintering grounds, often in mixed-species foraging flocks
Size: 0.14 m
Wingspan: 0.23 m
Lifespan: 5 years

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