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The most widespread member of its genus in North America, and the only Polioptila found in colder temperate regions, the Blue-gray Gnatcatcher breeds across a broad range – from Maine and southern Ontario south to Belize and El Salvador, and from New England west to northern California. This is also the only truly migratory gnatcatcher, although individuals breeding in Central America are largely resident.
These birds inhabit a wide range of wooded habitats but prefer moist areas with broad-leaved trees, often at or near habitat edges. They prefer to nest in open scrubby areas and forage in denser vegetation. They are active feeders that usually glean insects off foliage but also catch them by hovering and sallying after flushed prey. The calls and songs of this species are high-pitched and nasal. The sexes are only weakly dimorphic and pair monogamously. Males contribute significantly to nesting attempts, including nest construction, incubation, and the feeding of nestlings and fledglings. Nests are neat and cup-like, held together with webbing and decorated with lichens, and usually built well out on side limbs and branches of trees or large shrubs. Populations of this species have increased over the past 25 years, expanding northward, most dramatically in the northeastern U.S. and southeastern Canada.
Most recent studies of this species have focused on nest site characteristics and response to cowbird parasitism; results have indicated that parasitism can have severe consequences on annual productivity. Localized population declines may be related to nest failures resulting from parasitism. Future studies would do well to focus on habitat use and factors influencing productivity in localized populations, including the impacts of parasitism on population sustainability.