Bay-breasted Warbler (Setophaga castanea)
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. 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.
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.