Wilson's Warbler, Wilsonia pusilla
Wilson's Warbler, male, 6/10/10, Yuba Pass summit, Sierra Co
Wilson's Warbler. 6/8/10, Yuba Pass summit, Sierra Co
Wilson's Warbler, male, 4/15/11, Tilden Park, Berkeley, Alameda Co
Wilson's Warbler, female, 5/15/08, Galileo Hill, Kern Co
In some females, the cap is all-brown or olive, as above; other females have a black cap similar to those of males, on average smaller, but size overlaps. The birds shown in the top three pictures were singing and defending territory, and are certainly males; females tend to be less bright-colored overall, and I judge the bird shown below (with a black cap, barely visible) to be a female.
Wilson's Warbler, female, 4/15/11, Tilden Park, Berkeley, Alameda Co
This apparent female held this position for some seconds, while quivering its wings; singing males were all around. It looked to me like a courting display indicating receptivity, but interestingly the article on Wilson's Warbler in Birds of North America Online reports that the literature on the species as of 1997 indicated: "no evidence of display behaviors, including pre- or post-copulatory displays."
Wilson's Warbler, juvenile, 7/31/09, Upper Red Rock Lake campground, Centennial Valley, MT
This one is either a female or more likely (from the whitish feathers, apparently vestiges of down) a juvenile. Wilson's Warblers hold their juvenal plumage very briefly, for as little as a week, before they begin their first pre-basic molt.