Purple martins are the largest North American swallow, measuring 20.3 cm long and weighing 56 g on average. Their wingspan is about 45.7 cm. Males are a bit larger, entirely shiny, and deep purple or almost black in coloration. Purple martin males have less shine on their blackish wings and tails compared to their heads and backs. Females are overall gray or gray-blue with darker wings and crown feathers and feature a white breast smudged with varying degrees of gray. Western females are overall paler than eastern. Purple martins have a dark, black-brown bill and the average length is 8.2 mm for males and 8.5 mm for females. Their gape tends to be yellow in young birds and a dull orangey-brown color in adults. Their legs and feet have a black brown coloration and their eyes are dark brown. (Brown, 1997; Layton, 1972)

Juvenile purple martins are overall gray to black with a white belly and gray-streaked breasts. In juveniles, there is a much clearer line between their gray throats and white bellies compared to females which will have a primarily gray belly. In flight, a juvenile’s tail may have a narrow, slight fork whereas adult tails are distinctly forked and wider.(Brown, 1997; Layton, 1972)

Populations residing in the southwest United States exhibit lighter coloration than purple martins in other regions. This lighter coloration is hypothesized to be an adaptation to the desert climate and serves to absorb less heat.(Layton, 1972; Wade, 1966)