Slaty-backed Gull — Tullytown Landfill near Tullytown, Bucks County; December 21, 2007. Photo by Tom Johnson. Note the bird’s bulky body and darker mantle color and broader white crescent area than that of the surrounding Herring Gulls.

Slaty-backed Gull — Van Sciver Lake near Tullytown, Bucks County; December 22, 2007. Photos by Devich Farbotnik. Again, note the bird’s darker mantle color than that of the surrounding Herring Gulls. The dark smudge through the eye is barely visible here. This photo shows the bird’s wing pattern. The underside of the bird’s right wing is directly above the light head of the gull flying towards the camera. The upperside of the bird’s left wing is just to the left of that. Note the subapical spots (“string of pearls”) on the bird’s left wing and the broad, white trailing edge of both wings.