30 November 2016

November 2016 Quiz

Age: Pointed primaries, patterned wing coverts and some apparent juvenile scapulars assist in aging this gull as a 1st cycle individual. Most of the lower scapulars are juvenile (=1st basic), but the faint gray upper scapulars are non-juvenile (=formative or 1st alternate).

Identification: The uniform paleness to this bird suggests a large 4-year white-winger. Our white-wingers are Glaucous, Glaucous-winged, Thayer's and Iceland Gull. The mostly black bill pattern would be inconsistent with Glaucous Gull, especially one that has already renewed some scapulars. The primaries are too pale for a Thayer's Gull. This leaves us with Glaucous-winged and Iceland Gull.

A few features should immediately jump out at the observer as pro Glaucous-winged: This is a bulgy gull with a short wing projection and a long/strong bill. Iceland Gulls tends to show more petite bills, longer wings, and proportionally larger eyes placed lower on the face.

This month's quiz bird was identified as a 1st cycle Glaucous-winged Gull. This age group regularly shows variable scapular molt early in its hatch year. The molt can be absent to extensive. The bill pattern also varies from all-dark to paling around the base. Interestingly, some hatch year Glaucous-winged Gulls show this "mismatched" whitish head against a darker body. Whether this is due to bleaching, an early molt restricted to the head/neck, or just natural variation isn't clear.

January. San Mateo County, California.