A grayish warbler highlighted with rich red-brown and creamy buff in the summer, the Bay-breasted transforms itself in the fall into a mostly green and white warbler with a hint of bay on the flanks. Uncommon during migration, these birds are numerous in the forests of northern Canada, where they specialize on spruce budworms. They nest in low-elevation coniferous forests and forage fairly low in dense foliage on the inner part of the tree. In autumn they closely resemble Blackpoll Warblers, despite looking so unlike them in summer.
Most bird watchers’ best chance to see Bay-breasted Warblers will be during spring or fall migration. In spring, listen for the male’s song, which is similar to songs of Cape May and Black-and-white Warblers. The species forages at medium heights on the nesting grounds, but migrants frequent smaller trees and shrubs. In May, migrants may be numerous at hotspots in the eastern Great Lakes. In September, checking warbler flocks from Appalachia to the Atlantic should produce a few, but they are generally uncommon.
Bay-breasted Warblers are voracious predators of spruce budworms. One study concluded that they consumed over 13,000 budworms per hectare (about 2.5 acres) in just 41 days.
The density of nesting Bay-breasted Warblers during outbreaks of spruce budworm was very high in historical times. One Ontario study in 1946 found as many as 230 pairs in just 247 acres.
Young Bay-breasted Warblers in fall can look very similar to Blackpoll Warblers. One surefire way to distinguish them is by the color of the soles of their feet—bluish gray in Bay-breasted, yellow in Blackpoll.
Bay-breasted Warblers nest in much the same habitat as Cape May, Blackburnian, and Blackpoll Warblers, which also feed on spruce budworm, but the species all forage and nest at different heights. Bay-breasted Warblers place their nests an average of about 16 feet high. Blackpoll Warbler nests are on average 5 feet higher, and Cape May and Blackburnian nests are usually over 40 feet and sometimes as high as 80 feet.
Adult Bay-breasted Warblers appear to follow a more westerly migratory route in the fall than first-year birds. More adults migrate west of the Appalachian Mountains than east of them, while first-year birds are more frequent east of the mountains and along the coast.
In contrast to the more stable populations of other warblers, Bay-breasted Warbler numbers go up and down depending on outbreaks of the spruce budworm. The birds are abundant during infestations, but decline or even disappear from some areas a few years later. A similar relationship with spruce budworm exists for Tennessee and Cape May Warblers. Blackburnian and Blackpoll Warblers also readily consume budworms but are not as specialized in their diets.
The oldest known Bay-breasted Warbler was at least 4 years, 11 months old.
Warblers
