The adult's speculum color never changes, even though body color can change dramatically depending upon the time of year. These small birds weigh less than a pound. Their speculum is green, and the drakes have a rusty head, green eye stripe, and white belly. Drakes whistle, and hens quack softly. In flight, green-wings twist and turn and flash by singly or in compact, erratic groups of five to 50 birds.
Drakes have a low-pitched, rheeb call, and hens use a series of loud quacks.
They travel in loose flocks with some pairing evident. Wingbeat is stiff and not very rapid. Their long neck and tail and greenish-brown speculum are distinctive features. Drakes have a chocolate-colored head, white belly, and long, black tail. Drakes whistle, and hens call with a series of quacks.