Bay-breasted Warbler

Like the Tennessee and Cape May Warblers, the Bay-breasted is one of the famous “spruce budworm” warblers: species whose populations fluctuate dramatically in response to infestations of this forest pest. Spruce budworm is a moth whose caterpillars feed on fir and spruce needles, and it follows a 30–40-year cycle. The last big budworm outbreak lasted from 1967 to 1993 and is estimated to have affected 136 million acres. As the budworm outbreak subsided, populations of these warblers also declined, and they became relatively scarce as both breeders and migrants in New Hampshire.

But another budworm outbreak started in Quebec in 2006, and already we are seeing increases in Bay-breasted Warblers, especially during migration. This can be a tricky species to confirm during spring and summer unless you see it, since the high squeaky song is similar to those of those of Cape May and Black-and-white Warblers. If you see the singer however, there is no room for confusion, since the male Bay-breast is distinctly marked in black and rich chestnut. They molt out of this plumage in the fall, and southbound birds can be confused with both Pine and Blackpoll Warblers. Many fall birds will retain some degree of chestnut on their sides, but if they don’t another good field mark is their generally unstreaked buffy underparts. The corresponding areas on similar species tend to be white, grayish, and often show at least faint streaking.

Although widespread in northern New Hampshire, Bay-breasted Warblers are still not common. The highest numbers tend to be from the more extensive conifer forests in Pittsburg, the Lake Umbagog area, and parts of the White Mountains. They seem to prefer older trees but will shift into younger or mixed forests during budworm outbreaks. During migration they can occur anywhere, and in spring seem less partial to conifers than other species such as Cape Mays and Blackpolls.