Correct on both ID's & yes, many birds' bills change color as they mature. Also some bills are different color in breeding season vs. non-breeding season & there are some male vs. female bill differences also.

I'm thinking maybe a blue-headed vireo? I know the picture is pretty dark. But the eye ring and eye line and 2 wing bars make me think it might be the blue-headed.

Thanks MrBlubs, I've looked at a lot of images, and I'm thinking it's probably an immature common yellowthroat. We do have yellowthroats around. It just seems a bit large though.

Hmmm, no wing bar, spotted chest, eye line. Perhaps a palm warbler, but I don't feel real good about that ID. tj