Adult male Tree Swallows have iridescent blue-green upperparts, white underparts, and a thin black eye mask. Females tend to be duller in colour, with more green or brown colouring in their upperparts. Juveniles have completely brown upperparts. Unlike other swallows, Tree Swallows supplement their diet of insects with certain berries, especially those of Myrica shrubs.
Tree Swallows nest in natural or artificial cavities. They are unable to excavate their own holes, so they seek out old woodpecker holes or natural cavities in standing dead trees. Tree Swallows also readily use nest boxes, including those built for bluebirds.
Tree Swallows typically lay between four and seven white eggs. The female is responsible for most of the nest building, and incubating the eggs. Tree Swallows usually only have a single brood. A female may attempt a second brood if the nest fails early in the season.
Although this species has not yet been assessed by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC), there are some indications it is declining in eastern North America.