Black-throated blue warbler (Dendroica caerulescens)

Synonyms: Motacilla caerulescens
GenusDendroica (1)
SizeLength: 13 cm (2)
Weight9 - 10 g (3)

The black-throated blue warbler is classified as Least Concern (LC) on the IUCN Red List (1).

The black-throated blue warbler (Dendroica caerulescens) is a common migrant of the north-eastern United States and south-eastern Canada. Among the most sexually dimorphic of the songbirds, the male and female black-throated blue warbler differ strikingly in their plumage (3).

The male black-throated blue warbler is characterised by a deep blue head and back, with a black throat, mask and flanks, white underparts, and a prominent white patch at the base of the primary feathers. In contrast, the female has dull olive plumage with sandy underparts, a pale line above the eye, and a fainter or absent white patch on the primaries (4). The difference in appearance between the sexes is so conspicuous that early naturalists believed the females to be a separate species, referring to them as “pine swamp warblers” (3).

Unlike many New World warblers, the black-throated blue warbler does not alternate between a spring and autumn plumage, making it easily recognisable year-round (3). The species’ typical song is a series of four or five shrill notes ending on an inflection: “zwee zwee zwee zweee” or “I am laz-ee” (2). The call is an abrasive ‘dit’ (4).

The black-throated blue warbler spends the summer (May to August) in its northern breeding grounds in eastern Canada and the United States, from southern Quebec, Ontario, New Brunswick and Nova Scotia, to Georgia in the south and Minnesota in the west (4). It overwinters in the Greater Antilles and coastal regions of Central America (2).

During the autumn migration, the black-throated blue warbler may occasionally be spotted west of the Mississippi River, and it is occasionally seen in Florida during the winter (October to March) (3).

In its northern breeding grounds, the black-throated blue warbler is most abundant at higher elevations along the Appalachian range. This species prefers deciduous or mixed coniferous montane forest, characterised by dense underbrush, thickets of rhododendron (Rhododendron spp.) or mountain laurel (Kalmia latifolia), and bogs (3).

The black-throated blue warbler selects similar habitats in its overwintering grounds, including broadleaf and pine forests, shrubby secondary growth forests, and coffee plantations (5). During the spring and autumn migrations, the black-throated blue warbler can be found along forest edges, riparian woodlands, and in other densely vegetated areas, including parks and gardens (3).

In the early summer, male black-throated blue warblers establish territories and pair with females. Territories are advertised with singing and are aggressively defended, and females are closely guarded from other males. Although the black-throated blue warbler is generally monogamous during the breeding season, extra-pair copulations are common among both sexes (3).

Mated pairs of black-throated blue warblers can produce one to three broods per season, with clutch size ranging from two to five eggs (3). Incubation is performed by the female and lasts 12 to 13 days. The nest, constructed from bark, dried grasses and twigs and lined with fur, mosses or rootlets, is typically situated less than 1.5 metres above the ground (2), in dense foliage (6). Both the male and female black-throated blue warbler contribute to nest-building and to feeding the young, which fledge eight or nine days after hatching (3).

In the winter, the black-throated blue warbler is typically solitary and may occupy the same territory from one year to the next (2). Both males and females are generally intolerant of other warblers of the same sex. As this species does not sing during the winter months, exclusive territories are maintained through visual contact and avoidance, as well as chasing and calls (3).

The black-throated blue warbler is insectivorous, with flies, beetles, and insect larvae comprising most of its diet, although it also occasionally feeds on fruits, berries and seeds (2) (3). This species tends to forage in middle-level trees and underbrush (2), remaining within the boundaries of its territory (3). During its autumn migration, the black-throated blue warbler may be drawn to peanut butter or suet at bird feeders (2).

The black-throated blue warbler faces potential threats from deforestation and habitat fragmentation in its Neotropical wintering grounds. This species most commonly occurs in forested tracts of at least 100 hectares, and its reproductive success is higher near forest interiors than edges, suggesting that undisturbed woodlands may be critical to its conservation. However, several studies have revealed that the black-throated blue warbler is not affected by logging activity in its northern breeding grounds, as long as its habitat maintains a dense shrub layer (3).

Long-term climate change may potentially affect the migratory patterns of the black-throated blue warbler (3), and this species is also occasionally affected by brood parasitism by cowbirds (2). However, more research is needed to ascertain the severity of these threats (3).

Specific conservation measures are not currently established for this species, and it is not currently considered at risk of extinction (7). Although the black-throated blue warbler probably suffered declines following European settlement in North America, its numbers have increased in recent decades as fields and pastures are returned to forest (3).

Find out more about the black-throated blue warbler and its conservation:

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  1. IUCN Red List (March, 2011)
  2. Alsop, F.J. (2001) Smithsonian Handbooks. Birds of North America: Eastern Region. DK Publishing, New York.
  3. Holmes, R.T., Rodenhouse, N.L. and Sillett, T.S. (2005) Black-throated blue warbler (Dendroica caerulescens). In: Poole, A. (Ed.) The Birds of North America Online. Cornell Lab of Ornithology, Ithaca. Available at:
  4. Dunn, J.L. and Alderfer, J. (Eds.) (2006) National Geographic Field Guide to the Birds of North America. Fifth Edition. National Geographic Society, Washington, D.C.
  5. Wunderle Jr, J.M. and Waide, R.B. (1993) Distribution of overwintering Nearctic migrants in the Bahamas and Greater Antilles. Condor, 95: 903-933.
  6. Steele, B.B. (1993) Selection of foraging and nesting sites by black-throated blue warblers: their relative influence on habitat choice. Condor,95: 568-579.
  7. BirdLife International (August, 2011)