Wilson's Warbler, Cardellina 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

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 were 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. I think the bird seen from above in the fourth picture from the top is likely a male by the size of the cap and brightness of the plumage, but sexing these birds in Fall is more difficult.

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
The whitish feathers, last vestiges of down, mark this bird as a juvenile, while the strong yellow "eyebrow" or supercilium identify it as a Wilson's rather than a Yellow Warbler. Wilson's Warblers hold their juvenal plumage very briefly, for as little as a week, before they begin their first pre-basic molt.