Dendroica caerulescens

The black-throated blue warbler (Setophaga caerulescens) is a small passerine bird of the New World warbler family. Its breeding ranges are located in the interior of deciduous and mixed coniferous forests in eastern North America. Over the cooler months, it migrates to islands in the Caribbean and Central America. It is very rarely found in western Europe, where it is considered to be a non-indigenous species. The black-throated blue warbler is sexually dimorphic; the adult male has a black face and cheeks, deep blue upperparts and white underparts, while the adult female is olive-brown above and light yellow below. Predominantly insectivorous, the black-throated blue warbler supplements its diet with berries and seeds in winter. It builds its nests in thick shrubs and the closeness of its nesting sites to the ground make it a favored species for the study of warbler behavior in the wild. The black-throated blue warbler defends its territory against other birds of the same species for both nesting and winter habitats. As the black-throated blue warbler requires large, unbroken forest areas for nesting, its numbers are declining. The German naturalist Johann Friedrich Gmelin described the black-throated blue warbler in 1789. Its species name is the Latin adjective caerulescens meaning 'turning blue'. The black-throated blue warbler is one of the New World warblers or wood-warblers in the family Parulidae. This species was originally placed under the genus Dendroica. It was recently adjusted to be a member of genus Setophaga along with all other members of the genus Dendroica, based on the findings from a recent phylogenetic analysis of mitochondrial DNA and nuclear DNA in 2010. The old genus Dendroica was then deleted. Within the genus, it appears to have no particularly close relatives. The species breeds in North America and winters in the Caribbean. Some studies have observed significant differences in terms of migratory behavior and plumage color between northern and southern populations within the breeding range. The northern population mainly winters in the western Caribbean (Cuba and Jamaica) while the southern population usually spends the winter on eastern islands (Hispaniola and Puerto Rico). Moreover, males in the southern population have darker plumage than those in the northern population. These differences have led biologists to consider them as separate subspecies. However, a recent study in the United States reveals no significant genetic differentiation between northern (samples from Michigan, New Hampshire and New York states) and southern populations (sample from North Carolina). The study results actually show a recent population expansion from a single glacial refugium, therefore the current populations are homogeneous in terms of genetics. The differentiation that is observed between the northern and southern populations should have occurred quite recently. 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). 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.