A denizen of the forest interior, 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, and south at higher elevations of the Appalachian Mountains to northern Georgia. Most individuals migrate along the eastern seaboard from the Appalachians to the Atlantic coast, and overwinter in forested habitats of the Greater Antilles from Puerto Rico to Cuba, Jamaica, and Hispaniola, with some in the Bahamas, along the coast of the Yucatan and Belize, and a few in south Florida.
Males and females of this species differ strikingly in appearance, and were even considered separate species by early naturalists, including Alexander Wilson (Wilson 1810) and John James Audubon (Audubon 1841). Males are unmistakable in their blue-black-and-white plumage, while females are a more cryptic greenish-gray in color. This is one of the parulid warblers that does not molt into a “confusing fall plumage,” so individuals can be identified easily year-round. Populations differ somewhat in plumage, notably in the amount of dark streaking on the crown, dorsal feathers, and the size of white patches in wings—characters once used to separate the two subspecies.
Being a bird of forested landscapes, the Black-throated Blue Warbler likely experienced population declines when eastern North America was deforested and settled by Europeans in the 18th and 19th centuries (Whitney 1994). As fields and pastures in the heart of its range returned to forest mostly in the 20th century, its populations probably increased throughout the region. In the future, however, changes in the quality of its breeding habitat (Sillett et al. 2000, Rodenhouse et al. 2003, Sillett and Holmes 2005, Holmes 2011), overwintering habitat (Holmes 2007, see also Studds and Marra 2007, Studds and Marra 2011), and enhanced risks during migration (Sillett and Holmes 2002) could reverse that trend. Climate change is another factor potentially affecting the distribution and abundance of this species (Matthews et al. 2011), but thus far, the species seems able to adjust to warming temperatures ( Kaiser et al. 2014, Townsend et al. 2013, Townsend et al. 2016, Lany et al. 2016).
The Black-throated Blue Warbler is one of the most intensively studied migratory passerine species in North America, and research on its populations has led to a number of conceptual advances. It is one of the few migratory songbirds for which the demography of populations has been examined in both breeding and overwintering areas. 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 and non-breeding seasons (see reviews by Holmes 2007, Holmes 2011; Demography: Population Regulation and Limitation). In addition, survivorship has been assessed for oversummer (May–August), overwinter (October–March), and migratory periods (Sillett and Holmes 2002), which is unique for any migratory passerine. Also, the connectivity between breeding and overwintering sites for individuals and populations has been examined 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) and recently with geolocators (M. T. Hallworth, unpublished data).
Many other aspects of the species’ natural history, ecology, and behavior have been studied in varying amounts of detail, including 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 and nest-site selection (Steele 1992, Steele 1993, Holmes et al. 1995, Hahn and Silverman 2007, Betts et al. 2008a, Betts et al. 2008b, Goetz et al. 2010), foraging behavior and ecology (Holmes et al. 1979b, Holmes and Robinson 1981, Robinson and Holmes 1982, Robinson and Holmes 1984, Holmes 1986, Holmes and Schultz 1988), mating systems and paternity (Chuang et al. 1999, Chuang-Dobbs et al. 2001a, Chuang-Dobbs et al. 2001b, Webster et al. 2001, Kaiser et al. 2014, Kaiser et al. 2015, Kaiser et al. 2017a, Cline et al. 2016), sex-ratio bias (K. Grabenstein, MSW, unpublished data), and parental care ( Stodola et al. 2009, Stodola et al. 2010, Kaiser et al. 2014). Another extensive line of research has considered how this species responds to, and is affected by, environmental change, including that brought about by forest management practices (e.g., Bourque and Villard 2001, Harris and Reed 2001, Harris and Reed 2002b, Harris and Reed 2002a, Jones et al. 2003b, Betts et al. 2007, Cornell and Donovan 2010a, Cornell and Donovan 2010b, Chandler et al. 2012, Hache et al. 2013), invasive species ( Stodola et al. 2013), and a changing climate (Rodenhouse 1992, Rodenhouse et al. 2008a, Rodenhouse et al. 2009, Townsend et al. 2013, Townsend et al. 2016, Kaiser et al. 2014, Lany et al. 2016).
The extensive knowledge about the ecology and behavior of this species contributes to understanding of the factors and processes that affect the distribution and abundance of migratory passerines in general, and thus, to their conservation. However, much remains to be learned about its life history, population ecology, and responses to a changing environment.