This warbler is the eastern representative of the widely dispersed Setophaga virens superspecies, which includes the Golden-cheeked (S. chrysoparia), Hermit (S. occidentalis), and Townsend's (S. townsendi) warblers, and probably the Black-throated Gray Warbler (S. nigrescens) as well. This set of closely related species may date from periodic episodes of isolation imposed on ancestral populations by Pleistocene glaciations or even earlier periods of allopatry. The Black-throated Green Warbler itself is currently divided into two subspecies, the nominate race (S. v. virens), which occupies the vast majority of the breeding range, and S. v. waynei, a little-studied breeder in certain swamps of the southeastern U.S. coastal plain (Figure 1).
An abundant species with a distinctive, persistent song (one male sang 466 songs in an hour), the Black-throated Green is sometimes the most common breeding species of the northeastern coniferous forests. It also ranges southward into mixed coniferous-deciduous forests, or even entirely deciduous forests. Despite this broad range and abundance, its nesting biology has been little studied in recent decades and much remains to be learned about its migration.
Populations of this species are vulnerable to loss of preferred winter forest habitat in Mexico, Central America, and the West Indies. The Black-throated Green is a forest-interior species, and its numbers decline in forest fragments. Southerly populations breeding in eastern hemlock and Fraser fir are at risk as a result of wooly adelgids (Adelges tsugae) responsible for heavy mortality of these trees.
Some aspects of the Black-throated Green Warbler's breeding biology are well known, especially foraging ( Macarthur 1958 , Morse 1968a , Parrish 1995b ), population dynamics ( Morse 1976b ), interspecific interactions ( Morse 1971c , Morse 1977 ), habitat selection ( Morse 1976b , Parrish 1995a ), singing behavior ( Morse 1967c , Morse 1970b ), and recruitment to flocks at the end of the breeding season ( Morse 1970a ). This work has been largely conducted in coastal spruce forests in Maine. Regional differences occur in habitat selection ( Collins 1983c ; Parrish 1995a ), and foraging ( Rabenold 1978 ), and probably other life history variables as well. Many recent studies focus on the Black-throated Green's (and other species') response to silvicultural practices that fragment and modify habitat (e. g., Darveau et al. 1995 , Hagan et al. 1996 ; Thompson et al. 1999c ; Hobson and Bayne 2000b ). Most recent information on Black-throated Greens from the wintering grounds comes from studies of their participation in mixed-species foraging flocks ( Rappole et al. 1999 ; Greenberg et al. 2001 ).