Black-throated Blue Warbler

The black-throated blue warbler measures 13 cm (5.1 in) in length and weighs 8.4–12.4 g (0.30–0.44 oz). Wingspan ranges from 7.5–7.9 in (19–20 cm). This is a strongly sexually dimorphic bird. The adult male has white underparts with a black throat, face and flanks. The upperparts are deep blue. The immature male is similar, but with greener upperparts. The female has olive-brown upperparts and light yellow underparts with darker wings and tail, gray crown and brown patches on the cheek. Both sexes have a thin pointed bill and small white wing patches which are not always visible. Like many other warbler species, it has colorful plumage during the spring and summer. However, outside the breeding season its plumage is drab and less distinctive. In the fall, the black-throated blue warbler can be distinguished by its small white wing patches. Juveniles have brown upperparts with a creamy supercilium and brownish spots on the throat, the breast and the belly. The bird's song can be described as a buzzed zee-zee-zeeee with an upward inflection. Its call is a flat ctuk.

The black-throated blue warbler is a migratory species. It breeds in temperate mature deciduous forests or mixed coniferous-deciduous forest with a thick understory. The species is often found in hilly and mountainous regions in the northeastern United States and southeastern Canada. In late summer, it migrates to the tropical wooded and scrub habitats in the Greater Antilles for wintering. Along the migration route, the black-throated blue warbler can be observed in habitats such as parks and gardens. Its nesting site is more important than its foraging site in playing a role in determining the habitat of the black-throated blue warbler. The black-throated blue warbler is an open-nesting species, that nests very close to the ground so it has to choose a protected nesting site where the risk of predation is relatively low.

The black-throated blue warbler forages actively in low vegetation, sometimes hovering or catching insects in flight. It often forages in one area for a while before moving on to the next. It mainly eats invertebrates such as caterpillars, crane flies, and spiders. It may supplement its diet with seeds, berries, and fruit in the winter. Males and females prefer different foraging sites. While males usually hover among the higher shrub foliage between 3 and 9 m (9.8 and 29.5 ft), females tend to forage at lower strata. The time within a breeding season influences where the males forage. When it is time to feed the fledglings, males come down to the same foraging strata as females. The black-throated blue warbler mostly forages in the understory instead of the canopy. The large leaves and long branches in the understory affect its foraging behaviors. The black-throated blue warbler more often hovers rather than gleans its prey because it is more difficult to glean among thick understory foliage.