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QUIZ GULL #6 (answer below)


ANSWER:

The correct answer to Quiz Gull #6 is Glaucous-winged Gull or Glaucous x Glaucous-winged hybrid. It's a large first year bird, as evidenced by the nearly all black bill and mottled/checkered upperparts and lack of adult gray on the back.

The most noticable field mark is the rather pale primaries. They don't seem to be that way due to fading, as the primaries and tertials can be seen well, and don't appear to be worn. Fine patterns can still be seen in them. This shortens our list immediately to the more pale-winged gulls (in first year plumage): Glaucous, Iceland, Glaucous-winged, Thayer's, possibly Slaty-backed. The latter two should have primaries that appear more solidly brown. Glaucous only has black at the tip of the bill. That leaves "Kumlien's Gull" (considered here to be glaucoides Iceland x Thayer's hybrid) and a rather pale Glaucous-winged Gull as the main candidates. The bill looks rather small in the first photo and large in the second, demonstrating just how tricky a single photo can be. On closer inspection, I'd say the bill is rather large and bulbous. The eye appears rather small and beedy. The pattern on the coverts appears rather muddy, and not neatly checkered. The primary extension beyond the tail is quite short. All these characters point strongly to Glaucous-winged Gull and away from Thayer's/Iceland. Due to pale primaries and slight amount of pink at the base of the lower mandible, it's tempting to say this is a Glaucous x Glaucous-winged hybrid. I'd argue, based on the overall structure of the bird, that this is within range of Glaucous-winged Gull and have seen a number of such birds in Puget Sound. I suppose only DNA could prove my argument...

[Note, I've since changed my mind and am inclined to think there is Glaucous influence in this bird. The primaries are contrastingly paler than the rest of the body, and I'm inclined to believe that any gull with these fine subterminal marks on the primary tips has Glaucous or Iceland influence.]

It should be noted that Glaucous-winged Gull bills can be rather small in first winter, and only the largest males match Western in gonydeal bulge. The sketch of a Gl-W x Western bill in Kaufmann's Advanced Birding is a large extreme. Take a look at Figures 1 and 2 at this page for examples of smaller bills with little gonydeal expansion. Figure 2 is nearly within Thayer's range.

This photo was taken Dec 22, 1998 by Mike Rogers (copyright 1998) in Santa Clara County, CA.

This quiz had the lowest success rate of all, but the correct answer still managed to finish second. West coast birders had a clear advantage here, and did much better than others. I'll have to get away from that bias in the future...

GUESS # OF VOTES
Glaucous-winged Gull (25% correct) 6
"Kumlien's" or Iceland Gull 11
Thayer's Gull 4
Glaucous x Herring Gull 1
Glaucous Gull 1
Herring Gull 1