Very pale juvenile Herring Gull (Larus argentatus). The paleness and white rump are not typical of the American subspecies, L. a. smithsonianus. Viewed through telescope floating next to a smaller California Gull. The body had crisp white feather edges of scapulars and strong white marbling of wing coverts, typical of L. a. vegae from Bering Sea. Dark primaries in flight and elongated head shape rule out Thayer's Gull (L. thayeri). "vegae" is considered a true species by the BOU, and has not been formally accepted (even as a subspecies) for the Colorado state list.