Common and widespread throughout most of North America, breeding as far north as Alaska and wintering to Panama. A medium-sized swallow, with gleaming white underparts. Adult males are bright iridescent blue-green above; females and immatures are duller brownish with limited or no iridescence. Always note compact shape, fairly broad wings, and slightly notched tail. Occurs in a variety of open habitats including grassy fields, lakes, and marshes. Often in flocks, sometimes mixed with other species of swallows. Breeds in cavities, including human-made nest boxes. Listen for cheery gurgling calls. Compare especially with Violet-green Swallow; note Tree Swallow does not show extensive white wrapping around the cheek or the sides of the rump.