The Black-throated Blue Warbler inhabits large tracts of relatively undisturbed hardwood and mixed deciduous-coniferous forests in the northeastern United States and southern Canada, southward along the higher elevations of the Appalachian Mountains to northern Georgia. Most individuals migrate along the eastern seaboard from the Atlantic coast to and including the Appalachians, and most winter in forested habitats of the Greater Antilles from Puerto Rico to Cuba, Jamaica, and Hispaniolawith some in the Bahamas and a few along the Yucatan coast and Belize.
Males and females of this species differ strikingly in appearance, so much so that the two sexes were considered separate species by early naturalists, including Wilson ( Wilson 1810 ) and Audubon ( Audubon 1841 ). This is one of the paruline warblers that does not molt into a “confusing fall plumage,” so individuals can be identified easily year-round. Nevertheless, populations differ somewhat in plumage, notably in the amount of dark streaking on the crown, dorsal feather s, and wing coverts in malesa character once used to separate two subspecies, but now questioned. A bird of the forest interior, the Black-throated Blue Warbler probably suffered loss of numbers over the last 300 years from extensive deforestation during the settlement of North America by Europeans. In more recent decades, however, as fields and pastures in the heart of its range have returned to forest, populations have appear to be increasing. In the future, loss of forests in its Caribbean winter grounds could reverse that trend. The latter is supported by evidence suggesting that changes in the extent and quality of winter habitat in the eastern Greater Antilles may be affecting abundances in the southern part of the species' breeding range ( Rubenstein et al. 2002 ).
The Black-throated Blue Warbler is one of the most intensively studied passerine species in North America. It is one of the few migratory songbirds for which the demography of populations has been examined in both breeding ( Holmes et al. 1992 , Holmes et al. 1995 , Rodenhouse and Holmes 1992 , Sillett et al. 2000 , Sillett et al. 2004 , Sillett and Holmes 2002 , Sillett and Holmes 2005 , Rodenhouse et al. 2003 , Nagy and Holmes 2004 , Nagy and Holmes 2005a , Nagy and Holmes 2005b ) and wintering areas ( Holmes et al. 1989 , Holmes and Sherry 1992 , Wunderle 1992 , Wunderle 1995 , Sherry and Holmes 1996b , Sillett et al. 2000 , Sillett and Holmes 2002 ). Most of these studies have focused on the factors and processes that limit and/or regulate populations and where they operate, for example in breeding or non-breeding areas. In addition, survivorship has been assessed for the oversummer (May-August), overwinter (Oct-March), and migratory periods ( Sillett and Holmes 2002 ), an analysis that has thus far not been made for any other migratory songbird. Also, the connectivity between breeding and wintering sites for individuals and populations has been studied through analyses of stable isotope ratios in warbler tissues, mainly feathers ( Chamberlain et al. 1997 , Rubenstein et al. 2002 , Graves et al. 2002 , Bearhop et al. 2004 ). Studies have also been conducted on population fluctuations and regional synchrony in breeding areas ( Holmes and Sherry 2001 , Jones et al. 2003b ), range-wide age-ratios ( Graves 1997b ), breeding habitat use ( Steele 1992 , Steele 1993 , Holmes et al. 1995 ), mating systems and paternity ( Chuang et al. 1999 , Chuang-Dobbs et al. 2001b , Chuang-Dobbs et al. 2001a , Webster et al. 2001 ), responses to forest management practices ( Bourque and Villard 2001 , Harris and Reed 2001 , Harris and Reed 2002b , Harris and Reed 2002a ), and other related topics.