The Black-throated Blue Warbler (Setophaga caerulescens) is a small songbird native to North America and belongs to the New World warbler family, Parulidae. It is well-known for its striking coloration, unique song, and specific habitat preferences.

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Physical Description

Males:

Deep blue upperparts. Black face and throat. White belly and undertail. Small white wing patch, often used for identification in flight.

Females:

Olive-brown upperparts. Pale yellow or whitish underparts. Distinct white “eyebrow” (supercilium).. Dusky face and a subtle wing patch.

Size:

Length: 12–13 cm (4.7–5.1 in). Wingspan: 19–21 cm (7.5–8.3 in). Weight: 8–12 g (0.3–0.4 oz)

Sexual dimorphism is pronounced, with males much more colorful than females.

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Distribution & Habitat

Breeding Range:

Northeastern United States (Appalachians to New England). Southern Canada (Ontario, Quebec, Maritime provinces)

Wintering Range:

Caribbean islands, especially the Greater Antilles (e.g., Cuba, Jamaica, Hispaniola, Puerto Rico)

Habitat:

Prefers mature deciduous or mixed hardwood forests with dense understory. Breeds in moist, shady forests. Winters in tropical broadleaf forests, often in mountain regions.

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Migration

Migratory Species: Moves from North American breeding grounds to Caribbean wintering grounds.

Migration Period: Southbound: August to October. Northbound: March to May. Often solitary during migration and less likely to join mixed flocks than other warblers.

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Diet and Foraging

Diet: Primarily insectivorous; eats caterpillars, spiders, beetles, flies, and other arthropods. Occasionally consumes small fruits and berries in winter.

Foraging Behavior: Actively gleans insects from the undersides of leaves. Forages in the lower to mid-levels of the forest.

Employs “hover-gleaning” and short sallying flights.

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Breeding and Reproduction

Breeding Season: May to July.

Nesting: Cup-shaped nest built low in dense shrubs or saplings (often <1 m above ground). Constructed by the female using bark strips, moss, and spiderwebs.

Clutch Size: 3–5 eggs.

Eggs: White or cream with brown speckling.

Incubation: 12–13 days by the female.

Fledging: Young leave nest about 8–10 days after hatching.

Parental Care: Both parents feed the young.

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Vocalizations

Song: A distinctive, buzzy series of rising notes, often rendered as “zoo-zoo-zoo-zweee” or “I am so lazzzy”.

Call: A short, sharp “tsip”.

Males sing to defend territory and attract mates; song frequency declines after mating.

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Behavior

Territorial: Males are strongly territorial during the breeding season.

Solitary or in Pairs: Often seen alone or with a mate; less social than some other warblers.

Secretive: Tends to stay within dense understory vegetation, making it harder to observe.

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Conservation Status

IUCN Red List: Least Concern.

Population Trend: Stable to slightly declining.

Threats: Habitat loss from logging and forest fragmentation. Climate change may alter forest composition or migratory timing. Collisions with buildings and predation during migration.

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Interesting Facts

Sexual Dimorphism led early ornithologists to mistakenly believe males and females were separate species.

Philopatry: Males are known to return to the same breeding territories year after year.

Hybridization: Rare, but occasional hybrids with closely related warblers have been reported.

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Research and Study

This species is commonly used in avian ecology research, particularly studies involving:

Migratory connectivity. Territory and mating systems. Forest ecology and effects of fragmentation. Climate-related phenology changes