Possible Western Gull (Larimer County) (long!)

[Nick Komar]

Late this afternoon, hundreds of minnow-gorging gulls put on a spectacle along the west shore of Warren Lake, Fort Collins (Larimer County). Among the equinumerous Ring-billed and California Gulls were dozens of Herring Gulls, 3 Bonaparte's Gulls, and a handful of unusual individuals. Two were juvenal Thayer's Gulls. Two were probable 1st cycle Nelson's Gulls (Herring x Glaucous, identified by pale bodies, Glaucous-like black-ink-tipped bills, black wingtips and tails) and one I considered to be a possible 1st cycleYellow-footed Gull (species from North-West Mexico that winters in small numbers at the Salton Sea). I will look it up in my reference books to confirm, but first I will post the description here (for conclion, skip to next paragraph):

 

Gull observed floating and flying at 100-200 m through 7x binoculars and 20-60x Kowa telescope, for 10 minutes between 4:10 and 4:30 pm in great light as sun set behind me. When floating, similar in size and shape to a large, block-headed male Herring Gull (HERG); plumage matched juv Herring Gull with overall brown coloration (but lighter on head and neck). In flight also paler rump and whitish lower belly/vent. Crisp white edges to scapulars and wing coverts as in juv HERG. All black tail seen in flight. When floating, all black primaries and pale-tipped blackish tertials, darker than HERG. I did not view the color of the inner primaries relative to the outer primaries, which would be visible on the extended upperwing. Most notable feature of this bird was its bill, much larger and longer than that observed on nearby HERG, and pale-based (pinkish-horn color) with blackish tip beginning at the gonys, which was very angular. In my estimation this was the largest bill I have seen on a gull except for perhaps Kelp Gull, which would have a slightly deeper bill and thus not appearing quite as long as this bill. Unfortunately I did not have my camera, and the bird flew off by itself just before sun-down, headed southeast. All the other gulls eventually flew to the center of the lake between 4:45 and 4:55 to roost with the various ducks and grebes. Now to the reference books....

Sibley: Wow, definitely not a Yellow-footed Gull, nor Kelp, both of which are dark-billed and paler-rumped than this bird. However, a close match for first-basic Western Gull which can show a pinkish base to the bill after October. The bill did seem to droop as described by Sibley although I did not articulate that in my written description.

Olsen and Larsson: Same assessment, but bird still in juvenal plumage (no obvious sooty gray feathers yet; pale base of bill should develop beginning in December; northern population uniformly dark on belly whereas southern population paler on lower belly as described for this bird.)

Howell and Dunn: Similar assessment, close match to photographs of juvenal Western Gull (southern "wymani" population); also similar to juv. Yellow-footed Gull but these molt prior to November. Text describes increasing occurrence of "wymani" in inland locations, such as Salton Sea. Stress structural difference of Western Gull relative to Herring Gull (e.g. thicker wings) which was not noted. They also discuss extensive hybridization of Western Gull (especially northern population) with Glaucous-winged Gull.

 

Conclusion: I believe this gull is probably a Western Gull, but it needs to be photographed to assess structure and wing morphology more carefully. From what I noticed on the bird, hybrid status is not indicated, but can never be ruled out entirely. If it tuns out to be a hybrid, I would guess Herring crossed with either Western, Yellow-footed or Kelp. Birders should watch for a large brown first-cycle gull resembling Herring Gull with an enormous bill and document its appearance as much as possible as it may represent Colorado's first record of Western Gull. Hopefully it will stick around to permit more careful assessment. I would search for it and report back but I am migrating to the east coast in the morning, back in a week.

 

Nick Komar

Fort Collins CO