I can't seem to find this bird in any of my field guides. It's almost completely featureless, and every candidate I find seems to have more in the way of markings. Observed at Palo Alto on Saturday.
That was my first thought too Declan. What put me off was the very thin bill. Cowbird's bills are more conical. Could it be a female Brewer's Blackbird?
Thanks to both John and Declan for useful pointers. If it helps any, there was a group of maybe half a dozen (so not a solitary bird) all eating whatever they were finding on the ground. The habitat was the dried grasses you see in the photo, in the middle of a wetland area on San Francisco Bay. I've just looked up both the Brown-headed cowbird an the Brewer's Blackbird in Wikipedia. They both look close. I have a few more photos that are not with me at the moment, but I'll try posting a different pose later in hopes that it may offer more identifying information.
Thanks for the help. I've now been back to the field guides, helped by all your expertise, and I do think Brewer's Blackbird is it. The Brown-headed Cowbird has a red iris, according to Sibley, and this bird does not. Here's another photo giving a more head-on view. Again, thanks very much for the help. I was going cross-eyed looking at all the pictures of other birds.
I in the Brewer's camp too. In the original shot I thought it looked a bit stubbed tailed and billed for a Brewer's. I don't recall red iris for Borne-headed Cowbird, were you looking at Bronzed?