Black-throated Blue Warbler

The Black-throated Blue Warbler is a small wood-warbler that breeds in deciduous and mixed forests with a well-developed shrub understory across the eastern United States and southeastern Canada, wintering mainly in the Caribbean. Males are strikingly patterned in deep blue, black, and white, while females look entirely different, being olive-brown overall. Both sexes share a small white patch at the base of the primaries.

Breeding male body feathers are deep slate-blue on the crown, nape, and back, with a solid black face, throat, and flanks contrasting sharply against white underparts. Females and immatures lack the blue and black pattern, instead showing plain olive-brown upperparts and buffy underparts. In both sexes, a small but distinctive white spot at the base of the primary flight feathers, often called the "handkerchief," is a key identification feature not shared by similarly plain-looking warblers.

Adult males in breeding plumage show the classic blue, black, and white pattern, while adult females are a much plainer olive-brown with a pale eyebrow stripe. Nonbreeding males retain much of the same bold plumage, unlike many warblers that become drab outside the breeding season. Immatures resemble females but may show a less distinct wing patch. A single complete molt occurs after breeding, with limited feather wear affecting brightness before migration.

This species breeds in deciduous and mixed forests with dense shrub understory, particularly in the northeastern and Appalachian regions of North America. It winters primarily in forested habitats of the Caribbean, especially in mountainous areas of the Greater Antilles.

Black-throated Blue Warblers forage low in the forest understory, gleaning insects from leaves and twigs, often close to the ground compared to many canopy-foraging warblers. The male's song is a slow, buzzy "zee-zee-zee-zreee," rising at the end. Nests are built low in shrubs or saplings. The species migrates at night in a fairly direct route between breeding and Caribbean wintering grounds.