Spotted Sandpiper (Actitis macularius)

This one is has acquired breeding plumage showing the bold spots on its underparts. Males and females are identical in appearance, but this species is particularly interesting for sex-role reversal. Females are polyandrous, mating with up to four different males. Males do most of the incubation and caring for the young. The function of their characteristic "teetering" behavior in which their whole rear-end bobs up and down is one of the unsolved mysteries of Ornithology. It appears to have evolved independently in several unrelated birds such as waterthrushes. Teetering is known to increase in frequency when the bird is nervous or agitated but stops with alarm, courtship, or aggression.