I feel that this is a Cassin's Vireo Vireo cassinii; it was coming in to one of the drips at Paradise Pond, Port Aransas, Nueces county,
Texas, on September 16, 2012. The contrast between the dark neck-sides and the pale throat is somewhat distinct and not typical of Cassin's - but it is within range for Cassin's, as a search through images online will demonstrate. Everything else looks much better for Cassin's, I feel:
Can Blue-headed ever have the underparts as dingy as on this bird?:
Note that the upperparts are almost concolorous from the lower back onto the crown:
