Thanks, Frankie, for your response. I also got response from Lee Ann van Leer (Bird Academy Project Assistant, who often responds in these discussion groups) in a private email to me about a technical issue I was having on the website. In that email she also gave her opinion on this. She thought it was most likely a female Common Yellowthroat.
After checking Birds of the World, I've ruled out the Kentucky Warbler. As far as Common Yellowthroat and Nashville Warbler go, it looks to me like it could be either. In the case of the Nashville Warbler, it looks like it could even be a male - both sexes look very similar to me.
So far I can't decide.