Swan, largest waterfowl species of the subfamily Anserinae, family Anatidae (order Anseriformes). Most swans are classified in the genus Cygnus. Swans are gracefully long-necked, heavy-bodied, big-footed birds that glide majestically when swimming and fly with slow wingbeats and with necks outstretched. They migrate in diagonal formation or V-formation at great heights, and no other waterfowl moves as fast on the water or in the air.