“If we lavished any superlatives on the Tree Swallow—and our memory misgives us that we did—we regret it now. Not but that the Tree Swallow is strictly deserving—oh, a very deserving bird—but we needed all our superlatives for present use, and one hates to repeat. What shall we do for the Violet-green Swallows? Simply this: we will call them children of heaven.” (Dawson 1923)
Despite an extensive distribution, less is known about the Violet-green Swallow than nearly any other North American swallow. Recent studies of this species' breeding biology and behavior have begun to emerge, however; so current knowledge, previously based mostly on reports from the 1940s and 1950s (and involving observations of only one or a few pairs) is growing.
The Violet-green Swallow occurs principally in montane coniferous forests, often nesting in inaccessible sites such as abandoned woodpecker holes in tall dead snags. The species breeds primarily in western North America from central Alaska and western Canada south to the Mexican highlands, rarely occurring east of the Rocky Mountains. It winters mostly from Mexico south to Guatemala, El Salvador, and Honduras. Three subspecies are recognized, Tachycineta thalassina lepida, T. t. brachyptera, and T. t. thalassina, the latter two confined as breeders principally to Mexico.
Violet-green Swallows nest both solitarily and in colonies of up to 25 pairs. The birds are often highly gregarious during foraging, migration, and when away from their nests, and flocks of several hundred birds are commonly observed in these contexts. Females typically lay 4 to 6 eggs in an abandoned woodpecker hole, crevice in a cliff, or bird box, often returning to the same breeding site in successive years. Like most other swallows, Violet-greens feed exclusively on insects caught in flight, often at high altitudes.
This swallow appears to be the western counterpart of its congener, the Tree Swallow (T. bicolor). Although the two species are similar ecologically, they do breed sympatrically in certain portions of their range. Because the Tree Swallow appears more willing to use bird boxes and occurs in more accessible eastern locations, however, it has so far attracted more research attention from ornithologists.