Dendroica caerulescens
black-throated blue warbler

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, Rodenhouse, and Sillett, 2005)

Black-throated blue warblers are found in tracts of undisturbed deciduous and mixed-deciduous forests in their breeding range. Forests they occur in include those with maples (Acer), birches (Betula), beeches (Fagus grandifolia), eastern hemlock (Tsuga canadensis), spruce (Picea), and fir (Abies). The elevational range of these forests varies throughout the region. They prefer forests with a dense, shrubby understory. They migrate along woodlands and woodland fragments, including riparian forests. In winter they are found in tropical forests, including secondary forest, plantations, and disturbed forest fragments. (Holmes, Rodenhouse, and Sillett, 2005)

Black-throated blue warblers are about 13 cm long and from 9 to 10 g. Males and females have different plumages. 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 their dorsal feathers. They have black legs, feet, and bills, but they begin life with flesh-colored legs, feet, and bills. (Holmes, Rodenhouse, and Sillett, 2005)

The oldest recorded black-throated blue warbler was at least 10 years old. There is some evidence that older individuals may have higher rates of survival, or higher site fidelity. Survival rates in the winter range were from 66 to 77% for females and males, respectively. Nestling mortality is largely from predation but nestlings also die from exposure during cold or rainy weather. (Holmes, Rodenhouse, and Sillett, 2005)

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 aggressively defend territories for feeding and nesting, excluding all conspecifics from the territory. (Holmes, Rodenhouse, and Sillett, 2005)

Home Range

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. Although black-throated blue warblers don't seem to return to their natal site in their first year, adults seem to return to the same breeding and wintering sites each year. (Holmes, Rodenhouse, and Sillett, 2005)

Black-throated blue warblers use a series of calls and songs to communicate. Females vocalize sometimes, but males perform the majority of songs. Male songs vary with individual, but there are two main song types: 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." The first song type is the most commonly heard and varies substantially among males. Males use other kinds of songs as well, although their purposes and contexts are not well understood. 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. (Holmes, Rodenhouse, and Sillett, 2005)

Black-throated blue warblers are mainly insectivorous during the breeding season and supplement their insect diet with fruits during the winter. In the breeding range, these warblers eat mainly beetles, caterpillars, butterflies and moths, flies, bugs, and spiders. In the winter they eat as much as 95% insects, but supplement their diet with berries, other fruits, flower nectar, and honeydew excretions from scale insects. Black-throated blue warblers forage by themselves from 22 to 70% of daylight hours, depending on the season and their energy requirements. Females forage more during nest-building and the weeks leading up to egg laying, up to 70% of daylight hours. Males generally forage for 30-32% of daylight hours, but forage for an additional 20% when they are singing to defend nesting territories. They forage in undergrowth shrubs and forest canopy layers, taking most of their prey from leaves and bark. (Holmes, Rodenhouse, and Sillett, 2005)

Black-throated blue warbler adults are preyed on by birds of prey, such as Cooper's hawks (Accipiter cooperi). Eggs and nestlings are taken by a wide variety of nest predators, including sharp-shinned hawks (Accipiter striatus), blue jays (Cyanocitta cristata), red squirrels (Tamiasciurus hudsonicus), eastern chipmunks (Tamias striatus), martens (Martes americana), fishers (Martes pennanti), flying squirrels (Glaucomys), raccoons (Procyon lotor), black bears (Ursus americanus), and garter snakes (Thamnophis sirtalis). Black-throated blue warblers will mob predators and perform broken-wing displays to distract them. Parents give a high-pitched warning call when they see raptors and will respond to the warning calls of other birds. (Holmes, Rodenhouse, and Sillett, 2005)

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. Only 2 nesting records indicate parasitism: bot flies (Oestridae) in one and parasitic fly larvae (Calliphoridae) in another. Brown-headed cowbirds will parasitize the nests of black-throated blue warblers, especially in areas of disturbed forest. If parasitized, they can successfully raise a cowbird young about 60% of the time. (Holmes, Rodenhouse, and Sillett, 2005)

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, Rodenhouse, and Sillett, 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, Rodenhouse, and Sillett, 2005)

Black-throated blue warblers have a large range and large populations without evidence of significant population declines. They are considered "least concern" by the IUCN. They are considered sensitive to forest fragmentation, preferring areas of forest over 100 hectares in size, but they are found in disturbed forests and secondary growth, provided there is a lush understory. Similarly, in their winter range, black-throated blue warblers are found in a variety of forests, including disturbed forests, orchards, and plantations, but populations may be negatively impacted by habitat destruction. They are also found dead as a result of collisions with man-made objects, such as television towers, during migration. (Holmes, Rodenhouse, and Sillett, 2005)