These Oreothlypis warblers were all photographed at Paradise Pond in Port Aransas, Nueces county Texas on October 06, 2013 at the same exact spot within the space of about 10 minutes. I was surprised to find myself struggling to decide which species was involved with the first bird. NOTE: all the pics are as they came out of my camera (except for cropping/resizing and sharpening slightly) - the settings give the original pics a slightly green cast with slightly underexposed reds:
1) My tentative conclusion is that this is an extremely dull Nashville, mostly due to the unbroken eye-ring. I draw your attention to the pics and discussion of a bird photographed by Blake Maybank here. In this discussion a number of people said some version of: "other than the eye ring there is nothing that looks wrong for Orange-crowned", and some noted that the eyering was slightly broken at the front and back. Not the case here, but the vent seems very dull for Nashville and there appear to be blurry slight streaks on the breast, which is the brightest yellow part of the bird - all pointing more toward Orange-crowned, I feel.
So, a Nashville with extemely dull underparts and vent weakly yellow (how unusual would that be?), or an Orange-crowned with a complete eyering (how unusual would that be?): UPDATE: feedback via ID Frontiers produced a consensus that this is a Nashville Warbler:
2) to confuse things, there was this Nashville that seemed rather odd to me, with a strongly gray back, buffy tone to the lower throat and just a suggestion of a weak chest band - ?:
3) As a point of reference here is an obvious Orange-crowned Warbler: