It's hard enough to identify some adult birds in the first place. It gets a lot harder when you mix in the immature ones, which haven't developed their adult colors yet. We studied this bird - and a multitude of photo angles of it - for awhile before settling on identifying it as an immature American Redstart. I think the yellow blotches and the tail "design" are the identifying features.
A few days ago, I posted a photo of the adult version. It is quite a contrast.
(Click to enlarge the photos)