Sibley has Glaucous at just over 3 pounds and the rangier Great Black-backed at about 3 1/2. The Glaucous probably wasn't used to having to give ground.
The full adult flew right past.
There were a total of 4 Great blackbacks present. This one's subadult bill but near adult wings probably makes this a 3rd winter bird.
The heavy all dark bill and white rump of the first winter bird generally stand out as well.