This is a widespread sandpiper in North America breeding along the edges of freshwater wetlands. 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. Females are more aggressive, arriving on breeding grounds and establishing territories before the males arrive. The females then compete with each
other for males. Males do most of the incubation and caring for the young.
The "teetering" behavior of this species is found in several other sandpipers. It is also characteristic of the similar looking waterthrushes which are actually American warblers, although they act much like Spotted
Sandpipers. In "teetering" the tail and the whole rear end of the bird pump up and down as the birds forage along streamsides. The purpose of "teetering" behavior is one of the unsolved mysteries of Ornithology. It seems it probably has some selective advantage because it has evolved independently in a number of different species with similar habits, but this advantage is unknown.
I took this photo out of the window of the car. Canon PowerShot S3 IS 12x