Dull brown all over. Seems highly social. Can't identify song because of other birds around. Brewer's male has a yellow eye with dark pupil, no eyering. I agree that a flock of females is unlikely.
Brewers seems a bit large.
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.