Largest swallow in the U.S. Rather long notched tail and long, pointed wings. In the eastern U.S., nests exclusively in nest boxes and martin houses; also uses natural cavities in parts of the western U.S. Adult males are dark overall with a purplish-blue iridescence visible up close. Females are grayer with some patterning on the underparts; immatures have a white belly. Gives a liquid gurgling call. Winters in South America, where identification is complicated by several other extremely similar species of martin.