Here is another terrible bird photo that provides practice for our ID skills. We tend to get spoiled by the wonderful full-frame shots in birding magazines that show every detail. Birds don’t often look that good in the field. Remember, sometimes the best field mark is the lack of field marks. This photo was taken in Brattleboro, VT, but the species occurs throughout most of North America. Leave a comment with your ideas, and how you reached that conclusion.
Ethical Birder 
I’m going with black oak, John!
What? There’s a bird in there somewhere?
Oh, okay. I see a yellow vent and maybe greenish upper parts. The clues are that the undertail coverts are extremely long, nearly extending to the end of the very short tail, which seems to lack tail spots. I am sure that points to a warbler. The choices seem restricted to Yellow Warbler or one of the Opornis warblers.
I thought warbler, but have no idea which one. Warblers are still hard for me. I like quizzes though!
The key word here is yellow, bright yellow below and dingy yellow above. We can’t see much of the upperparts, but what we can see is basically unmarked. As Greg points out, the undertail coverts are very long, covering most of the underside of the tail, so that eliminates the long-tailed warblers like Wilson’s. The Opornis warblers (Kentucky, Mourning, Connecticut, and MacGillivray’s) all have this structural feature, but they also have gray or black on the head and some markings around the eyes. This leaves us with Yellow Warbler
The field guide to warblers by Jon Dunn and Kimball Garret has a wonderful set of plates that show warbler butts from below. This is often the only view we get of a warbler, but most field guides do not show the unique field marks found on this area of the bird.