This swallow was photographed in Maryland near the Potomac River this past weekend. It is in an area where tree swallows are very common, but it looks so different in color, and has none of the clearly defined color delineation between light and dark that is so apparent on the tree swallows. Maybe its just a trick of the dawn llight?
My guess is Northern Rough-winged Swallow. It could be a Bank Swallow also, but it doesn't seem the right tone to me. Do you have an image of the breast? That would help in the ID.
The lack of contrasting markings around the head and the buff-grey sides indicate Northern Rough-winged Swallow as Dan suggested.
Jim- A Cliff Swallow would show brick red around the face and swifts are unable to perch like this on a branch- their small feet and legs only allow them to latch on to the sides of rough surfaces like walls, trees and the inside of chimneys (although now I mention this, someone is bound to tell me they've seen a swift perching!). Swifts and swallows are unrelated yet look superficially similar due to convergent evolution. They both share a similar lifestyle of chasing and eating insects while flying and evolution has produced similar "answers" to this "problem" in both groups.