Bay-breasted Warbler

The Bay-breasted Warbler is a striking boreal-forest songbird whose numbers rise and fall with outbreaks of spruce budworm, its favorite prey. In breeding plumage, males are unmistakable: a rich chestnut ("bay") crown, throat, and flanks contrast with a black face mask, a pale buffy patch on the side of the neck, and a streaked blue-gray back.

Females are much duller, showing only a wash of bay color on the flanks and a greenish-gray head, but they retain the same overall pattern in miniature. By fall, both sexes molt into a plain, nondescript greenish-olive plumage with whitish underparts, making autumn identification one of the trickier challenges in warbler-watching.

Key field marks

• Breeding male: chestnut crown, throat, and flanks; black face mask; buffy neck patch; two white wingbars
• Female: much paler version of the male pattern, with only a faint bay wash on the flanks
• Fall/immature birds: plain greenish-olive above, dull whitish below, with faint wingbars and dark legs and feet
• Stocky build with a fairly heavy, slightly downcurved bill for a warbler

Similar species

Fall Bay-breasted Warblers are notoriously similar to fall Blackpoll Warblers. The best distinctions are leg color (Bay-breasted has entirely dark legs; Blackpoll has pale yellowish legs and feet, especially the feet) and underparts (Bay-breasted is cleaner and less streaked below, and often shows a buffy wash on the flanks and undertail coverts, whereas Blackpoll shows fine streaking on the breast).

Bay-breasted Warblers breed almost exclusively in mature boreal spruce-fir forest across Canada and the northernmost United States, where their population can boom locally during spruce budworm outbreaks and crash again once the outbreak subsides. During migration they pass through a broad swath of eastern North America, favoring mixed and deciduous woodland canopy, forest edges, and shade trees in parks.

They are long-distance migrants that winter in the forest canopy and edge habitats of Panama and northern South America, particularly Venezuela and Colombia.

Behavior and voice

Bay-breasted Warblers forage methodically in the mid to upper canopy, gleaning caterpillars and other insects from foliage and twigs; during budworm outbreaks they can consume enormous numbers of the pest caterpillars. The song is a very high, thin, and sibilant "seetzy-seetzy-seetzy-seetzy," pitched so high that it can be difficult for some listeners to hear, and it closely resembles the songs of Blackpoll and Cape May Warblers.

Nesting

Nests are built by the female, a loose cup of twigs and grass placed on a horizontal conifer branch, usually well up in a spruce or fir tree. Clutches typically hold 4-6 eggs, and pairs may raise more young when caterpillar prey is abundant.