The American Coot (Fulica americana) is a dark gray waterbird with a white-ish beak and yellow feet. It can often be found in the Arboretum and other watery spots in and around Davis. According to a professor of Avian Sciences 13, it is often highly aggressive, particularly towards other birds.
The American Coot, Fulica americana
Poor photo of a coot, but showing it's yellow feet.