The Northern Black-bellied Whistling Duck is a bit unusual, as far as ducks go. For one, males and females look alike. Next, they have long legs and necks, and prefer a very upright stance. There's more! They are sometimes called Black-bellied Tree Ducks because of their most unduck-like habit of loafing in trees (where it also nests). And they form strong monogamous bonds, like Geese and Swans. Why the name "Whistling Duck"? Because these are noisy birds which utter a clear whistling call which sounds something like "waa-chooo". Strange ducks indeed.