These transitional buntings are really tricky!
Rustic and Tristram's would have a supercilium that was as clear as the submoustachial stripe.
Common Reed Bunting crossed my mind but as there is a suggestion of a pale mantle stripe and I don't think odd habitat on migration is a justified problem, but I am not of aware of races with a two tone bill..
Whether juvenile Chestnut Bunting can look like this is beyond my experience, I would be surprised if the lateral throat stripe could be this prominent, but I like to hear other peoples opinion on this. Of course in the field you know you can rule Chestnut Bunting out if you saw white on the outer tail feathers.
There's a suggestion of grey at the sides of the neck - if this isn't an artefact or misplaced downy feathers, with the two tone bill, bubble gum pink legs and general jizz I think it's probably a Black faced Bunting.Not sure what else it could really be, but I'm open to suggestions!
Ric