I've got a couple more pictures that I unfortunately can't upload, but it looks like it's got the thick bill of a finch, black on top and yellow on bottom. Its chest is totally yellow, and it has a duskier yellow on the back that's not quite olive, and olive/brown wingtips and tail feathers that are outlined in bright yellow.
I originally thought it was a yellow warbler, but the beak is all wrong for it, so I'm not sure what I'm seeing unless someone let a saffron finch loose...













