It's a Yellow-headed Blackbird. Although common, they are somewhat rare in California. Sierra Valley is one of the few places they breed in N. CA. They are spectacular looking birds, although they have a horrible breeding song, but one that let's you know you are unmistakeably in a marsh. The female yellow-headed blackbird does indeed look much like a cowbird. Here is a close up of one:
http://www.pbase.com/imagestruck/image/126075110
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