This is a tough one for the ol' dufus.
I'm inclined to consider either a female Tennessee warbler or
an immature N. Parula if you consider that splash of highlight
as wing bars. They don't quite look like wing bars and Tennessee
are barless. Barless is a word, right? I almost wrote braless because
that just feels right even though I'm certain that isn't a real word either.
Anyway, if they are wing-bars then it might be a Parula that has
not yet developed those definitive Parula features. The angle of your
shot confounds appraisal. The really sad thing is that I'm beginning to
believe this drivel. SOMEBODY PLEASE STOP ME!
Ya know, that bill might actually have a hook. A better look at that
hook might prove to be a vireo after all. For some reason I can't see
the image anymore so I'm probably just beginning the lose it.