Seems highly social. Can't identify song because of other birds around. Cowbird beak shorter, broader. Brewers seems a bit large. Flight is straight and direct, with constant rapid wing beats.
Wings appear somewhat triangular, and comparatively short, in flight.
Thanks for the guesses, Steve. I'm still stumped. With your description, these are our first choices. Calif. One flew into my office and I caught it. Dull brown all over.