This most widely distributed and abundant swallow in the world is familiar to birders and nonbirders. Polytypic (6–8 ssp.; erythrogaster breeds in North America). Length 6.8".
Adult male: deep iridescent blue crown, back, rump, and wing coverts; deeply forked tail with large white spots; rich buff to rufous forehead and underparts; iridescent blue patches on sides of breast, sometimes with very narrow connection in center. Wings and tail black. Adult female: similar to male but with paler underparts and less deeply forked tail. Immature: duller above than adults, with shorter but still deeply forked tail, buffy throat and forehead, whitish underparts. Flight: very pointy wings, deeply forked tail, and low zigzagging flight are distinctive. Hybrids: rarely reported. Cliff and cave swallows in North America, and common house-martin in Old World.
Eurasian subspecies (rustica), casual in Alaska, has clean white underparts with broader dark breast band. East Asian subspecies (gutturalis) similar to rustica but smaller, usually with incomplete blue breast band.
No other swallow in North America shows such a deeply forked tail. Shorter-tailed juveniles in flight may suggest the tree swallow but will always show partial dark breast band and buff throat, white spots on tail.
Call: in flight repeats a high-pitched, slightly squeaky chee-jit. Song: a long series of squeaky warbling phrases, interspersed with a nasal grating rattle.
Status and Distribution
Common. Breeding: in various habitats in lowlands and foothills with nearby open areas and water. Former natural nesting locales, including caves and cliff faces, have now mostly been abandoned in favor of a variety of man-made structures, including barns. A cup-shaped bowl made entirely of mud and the bird’s saliva; 3–7 eggs (May–June). Migration: in spring, arrives in extreme southern United States. late January–early February, peaking in mid-May in northeastern United States. Early arrival in Alaska in mid-May. Departs northeastern United States. as early as mid-July, peaking late August–early September. Main routes through Central America and through Caribbean, but also a trans-Gulf migrant. Winter: often in fields and marshes mainly in lowlands; rarely in southern United States. Uncommon from Mexico south through Central America. Most common throughout South America as far as central Chile and northern Argentina; breeding noted in Buenos Aires. Vagrant: North American subspecies casual or accidental in western and northern Alaska, and Hawaii, southern Greenland, Tierra del Fuego, Falkland Islands. Eurasian subspecies (rustica) casual in western Alaska, Northwest Territories, Nunavut, and southern Labrador. East Asian subspecies (gutturalis) casual in western Alaska and accidental in British Columbia.
Common worldwide. In North America, the breeding range has been expanded by using man-made structures for nesting.
—From the National Geographic book Complete Birds of North America, 2006