Illustration by H. Douglas Pratt and Jonathan Alderfer
The blue-gray gnatcatcher is active, often foraging in trees or shrubs. Polytypic. Length 4.3"(11 cm).
Identification Thin and long tailed, with outer tail feathers almost entirely white (tail from below looks white). The bill is thin and pale gray. Breeding male: blue-gray above, including most of head and back. Crown has a black line at the forecrown that extends along the sides of crown; white eye ring contrasts with gray face. Wings brownish gray; tertials blackish, edged white. Underparts entirely white. Nonbreeding male: black on crown absent, resulting in grayish crown. Female: like nonbreeding male but grayer above.
Geographic Variation At least 7 subspecies; 3 north of Mexico. Nominate caerulea more extensively white tail; western obscura has a black base to the outer rectrices that extend beyond the undertail coverts. Western male slightly less blue on the back, with black forehead mark that is thicker, and less like a supraloral line found in nominate caerulea. Western females are dingier above; easterns are gray. A third subspecies, deppei, from south Texas, is smaller and perhaps paler on average, but distinguishing these characteristics is likely impossible in the field.
Similar Species Most confusion is likely to occur with black-tailed, and to a lesser extent, California gnatcatcher. These species have different calls; California is also darker below. The best feature is the tail pattern. Blue-gray is almost entirely white on outer rectrices; black-tailed and California have mostly black outer rectrices with white tips or edges. Be aware that in late summer gnatcatchers molt their tails. blue-grays will look mostly dark from below when their outer rectrices are dropped. See black-capped gnatcatcher.
Voice Call: a querulous pwee or various mewing calls. Western birds have lower, harsher notes, more like wrens; easterns’ common call slightly more wiry, thin. Song: thin, wiry notes; lower and harsher in western populations.
Status and Distribution Common. Breeding: various woodlands. Migration: spring migration begins in southern states in late Feb. Earliest migrants reach Great Lakes in late March, typically in early April. Peak late April through early May, with stragglers to later in the month. Fall migration starts as early as late June or July, in southern states. Farther north migration starts mid-August, with peak mid-September. Small numbers seen into October. Vagrant: annual in fall in small numbers to Atlantic Canada, August–November. Casual in spring to Atlantic Canada; to British Columbia and Ottawa in spring and fall. Winter: southern United States, south to Guatemala and Honduras.
Population Northward expansion in northeastern United States and southeastern Canada occurred in the 20th century.
—From the National Geographic book Complete Birds of North America, 2006
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