purple martins are a kind of swallow, of the genus "progne". like other members of this genus, they are larger than most of the other swallows. the average length from bill to tail is . adults have a slightly forked tail. adult males are entirely black with glossy steel blue sheen, the only swallow in north america with such coloration. adult females are dark on top with some steel blue sheen, and lighter underparts. subadult females look similar to adult females minus the steel blue sheen and browner on the back. subadult males look very much like females, but solid black feathers emerge on their chest in a blotchy, random pattern as they molt to their adult plumage.