Much of the Tree Swallow's natural history is influenced by the limited availability of suitable holes for nesting. Along with a closely related western species, the Violet-green Swallow (T. thalassina), Tree Swallows are the first swallows to return in spring. They may return as early as late March, when breeding grounds are still cold and snow-covered, to begin searching for nest sites. A lack of suitable nest sites can limit the population of Tree Swallows in an area.
Unlike other swallow species that nest in tight colonies, Tree Swallow males establish a territory close to 15 meters around a suitable hole. Males defend these sites aggressively, not only against other Tree Swallows, but at times against other hole nesting species as well. The natural nesting habitat was originally found around beaver ponds and wetland margins where standing dead trees were excavated by woodpeckers. Nest boxes, or bird houses, provided by humans enabled Tree Swallows to expand to treeless regions, where in some areas they have increased their populations. In others, the loss of natural habitat led to population declines.
Tree Swallows are a common species with an extensive range. They are found across North America north of central Tennessee in the East and north central New Mexico and California in the West. Their range in summer extends as far north as the tree line. When boxes were provided some 60 miles north of the tree line in northwestern Canada, Tree Swallows found them and bred.
Tree Swallows are medium-sized swallows with pure white underparts and slightly forked tails. Adults in breeding plumage have glossy metallic blue upperparts tinged with green. In fall and winter, the color of the upperparts is more green than blue. The sides of the head are colored blue-black. Wings and tail are sooty black with a faint green sheen. The bill is black, the eyes are dark brown, and the legs and feet are light brown. Adult females are slightly duller than males and may have brownish foreheads.
