I’ve tried Whatbird.com and there are a lot of small birds that look like this, but none have exactly the markings that you see in this picture , specifically the yellow markings. The most distinctive marking is a bright yellow patch on the top side of the tail where the tail meets the body. This is a very small bird, it would fit in the palm of your hand.
Looks like an immature Yellow-rumped Warbler. Note that they are somewhat variable in plumage.
Here are a couple more photos.
http://www.1000birds.com/images/Yellow-rumped-Warbler9.jpg
This one shows the yellow crown, which is not always apparent:
This one is the western form, “Audubon’s Warbler,” which has a yellow throat. Eastern ones have a white throat.
http://www.davesbirds.com/DavesBirds/images/Yellow-rumped%20Warbler_112705_5902.jpg
Here’s an adult male:
http://www.photobirder.com/Bird_Photos/yellow_rump_warbler_m.jpg
Yeah, that’s it. Thanks Colibri.
Darn it, Colibri, you always get to the bird questions before I can give it a go! Are you planning on going out of internet contact anytime soon?
(Just kiddin’ - I’ll certainly be coming to you if I get stumped!)
Ya gotta be quick to beat a hummingbird.