Let's try to get oriented. Obviously we are looking at a waterbird, whether it is a gull, loon, or tube-nose may not be immediately obvious. Still, no north american gull looks this way, and the legs are awfully far back on the body for any Larid. Similarly, no loons in basic plumage are not generally this brown on top, but I won't get into the structural features that rule them out.
Most people got that this bird was a tube-nose, though, and more specifically a shearwater. All dark shearwaters are ruled out as this bird is white underneath. Manx and Audubon's are much blacker on top and white-flanked.
Truthfully, though, there are a few pieces of information here that give away the answer: 1) there is an obvious white bar moving up onto the nape, although the entire nape is not visible; 2) dark flanks; 3) combination white uppertail coverts and dark rump; and 4) dull/darkish legs. Everyone who got it to tube-nose answered either Cory's or Great. Cory's Shearwater does not generally have any of these four criteria. The rump is generally a little paler than the body, even though it has white uppertail coverts. This gives Great Shearwater a white band above the tail, which is less pronounced on Cory's. Cory's Shearwaters also have bright pink legs, and clean, white flanks, unlike this individual. This bird is a Great Shearwater. Here is another crappy, though more discernible, photo of the same bird:
Mark (Mad Dog) Dettling
Thanks for playing!
By Luke Musher