American Wigeon (Anas americana)
People think wigeons change their migration patterns because the size of their yearly local population varies more than that of most ducks. Since the wigeon population covers such a wide area, if there are changes in its food supply, it can settle somewhere else for winter or summer and still find something to eat. Wigeons are found in the bay area during the winter months; they may be seen waddling across the lawns of Golden Gate Park to beg for food.
(Sources: The Sibley Guide to Birds, David Allen Sibley, Knopf, 2000; Cornell Lab of Ornithology)