Brewer's male has a yellow eye with dark pupil, no eyering. I agree that a flock of females is unlikely. The bird was released within a minute or so. 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.