Black-throated blue warblers are about 13 cm long and from 9 to 10 g. Males and females have different color patterns. Males have dark blue backs and black faces, throats, and sides. Their bellies and breasts are white. Females are olive green with buffy yellow throat, breast, and bellies. Females have a buffy eye stripe, a white semicircle below the eye, and a small white wing spot. Immature males have a greenish tinge to the feathers on their back. They have black legs, feet, and bills.
Black-throated blue warblers are found in northeastern North America in the summer, breeding season. They are found from the northern Great Lakes region east to the Canadian maritime provinces, throughout New England, and south through the Appalachian mountains. In winter they are found in southernmost Florida, the Antilles south to Trinidad, and the coastal Yucatan peninsula, from Mexico and Belize to Honduras. (Holmes, et al., 2005)
Black-throated blue warblers are found mainly in areas of undisturbed deciduous and mixed-deciduous forests in their breeding range. They prefer forests with a dense, shrubby understory. In winter they are found in tropical forests, including secondary forest, plantations, and disturbed forest fragments.
Black-throated blue warblers form bonds between males and females each breeding season. Sometimes males try to have two female mates. Pairs are formed very soon after arriving at the breeding site. Males guard their mates closely.
Black-throated blue warblers start breeding in late May and can breed into August. Females find nest sites and build nests out of strips of bark, cobwebs, and saliva. They line them with softer materials, like moss, hair, pine needles, or shredded bark. Females usually lay 4 white, speckled eggs in a clutch. Most females lay 2 clutches a year, either after losing a clutch to predators or after having successfully raised a brood. Incubation takes 12 to 13 days and young begin to fly between 8 and 10 days after hatching. They leave the nest at that point, but remain nearby and are fed and protected by their parents for another 2 to 3 weeks after they have begun flying. Black-throated blue warblers can breed in their first year after hatching.
Females incubate the eggs and brood hatchlings. Males will feed females while they are on the nest. Young hatch with their eyes closed and naked. Their eyes open at about 4 days old and they leave the nest at 8 to 10 days old, when they are just beginning to learn to fly. Males and females both feed nestlings and fledglings for up to 3 weeks after they leave the nest. Both parents protect their young from predators with alarm calls and by trying to distract predators by pretending to have a broken wing.
The oldest recorded black-throated blue warbler was at least 10 years old.
Black-throated blue warblers flit among vegetation and can hop on the ground. They are migratory and active during the day. They spend much of their time foraging, except when females are incubating eggs or brooding young, when they spend 75% of their time incubating or brooding. They are solitary throughout the year, except for mated pairs during breeding season. Males defend territories for feeding and nesting.
Foraging and nesting territories are from 1 to 4 hectares in size during the breeding season. In winter, foraging territories are from 0.2 to 0.3 hectares for males and slightly smaller for females.
Black-throated blue warblers use a series of calls and songs to communicate. Males do most of the singing. The two most common songs are: 1) a song of 3 to 7 buzzing notes that trills upward at the end, sounding like "zee-zee-zee-zreeee," and 2) a song of 2 to 5 notes that descends at the end, sounding like "zee-zee-zhurrr." Most songs are used during the breeding season, but there is some singing during migration and in winter. Males sing from perches in their home range.
Black-throated blue warblers eat mainly insects, but they will eat some fruits during the winter. They eat mainly beetles, caterpillars, butterflies and moths, flies, bugs, and spiders. Black-throated blue warblers forage by themselves among leaves and on branches. They take most of their prey from leaves and bark.
Black-throated blue warbler adults are preyed on by birds of prey, such as Cooper's hawks. Eggs and nestlings are taken by a wide variety of nest predators, including sharp-shinned hawks, blue jays, red squirrels, eastern chipmunks, martens, fishers, flying squirrels, raccoons, black bears, and garter snakes. Black-throated blue warblers will mob predators and perform broken-wing displays to distract them.
Black-throated blue warblers are important predators of insects in their forest habitats. They may also help to disperse seeds of the fruits they eat. There are few reported parasites in black-throated blue warblers. Brown-headed cowbirds will lay their eggs in the nests of black-throated blue warblers, especially in areas of disturbed forest.
There are no known adverse effects of black-throated blue warblers on humans. However, along with many other bird species, they carry West Nile virus. (Holmes, et al., 2005)
There is no direct positive impact of black-throated blue warblers on humans. However, they are lovely and interesting members of native faunas and may attract bird watching interest. (Holmes, et al., 2005)
Black-throated blue warblers are not considered threatened currently. Although they may be sensitive to forest destruction and prefer large, undisturbed areas of forest, they seem to tolerate disturbed forests in their breeding and winter ranges.
Tanya Dewey (author), Animal Diversity Web.
Holmes, R., N. Rodenhouse, T. Sillett. 2005. Black-throated Blue Warbler (Dendroica caerulescens). The Birds of North America Online, 87: 1-20. Accessed April 18, 2009 at http://bna.birds.cornell.edu.proxy.lib.umich.edu/bna/species/087.