The more rounded head shape and centered eye placement, and the square tail tip, suggest Sharp-shinned; but the leg looks to me more like pencil-thin than toothpick-thin, suggesting Cooper's. Other commentators noted the relatively broad terminal tail band, and the rufous shade of the head as favoring Cooper's. Online comments split 5-5 between the two species. See http://www.pbase.com/image/118737990 for another photo of this bird.