This gull was spotted at Valmont Reservoir, Boulder County, Colorado (USA) late in the afternoon on 8 January 2011 by Bill Schmoker during an organized field trip. It was seen by dozens of people. First impression was that of a young Great Black-backed Gull (Larus marinus, GBBG) due to whiter head and upper chest, larger size than accompanying Ring-billed Gulls (L. delawarensis), paler upperparts than other first-cycle black-backed gulls, and heavy dark bill.
Further inspection revealed very broad dark tail band (too thick for GBBG), solid gray-brown on belly (too much for GBBG), lack of barring on secondary coverts causing the effect of a evenly-colored pale gray-brown panel on the folded wing (reminiscent of Glaucous-winged or Slaty-backed gulls), obvious pale window (inner primaries) on the extended wing (suggestive of Herring Gull group).
Field marks that were wrong for American Herring Gull (L. argentatus smithsonianus) included whitish rump, white vent and lower flanks (contrasting with gray-brown belly), whitish base of tail, obvious barring on upper scapulars (like first-basic Iceland and Vega gulls).
When standing on the ice, the gull had a pot-bellied look like Glaucous-winged Gull (L. glaucescens, GWGU) or Slaty-backed Gull (L. schistisagus, SBGU). When sitting, blackish primaries extended well beyond tail.
On the extended wing, the outer primaries were mostly dark, but inner webs were pale, like SBGU. Secondaries were dark with white tips, forming an obvious secondary bar. In SBGU, the pale window extends to the inner secondaries, but I do not see that in this bird.
I believe the most likely explanation for this odd combination of field marks on a first cycle gull in early January is a hybrid Slaty-backed Gull x Vega Gull (L. vegae), with either 50% Vega genes (F1 generation hybrid) or 25% Vega genes (F2 backcross with SBGU). It could even be an F3 backcross with 12.5% Vega genes. This hybrid combination is apparently not unusual in northeastern Siberia in the breeding range of these two gull species, and it is likely that these hybrids would still be fertile. In Colorado, both species are considered accidental. Vega Gull, treated as a subspecies of Herring Gull by the A.O.U., is not yet accepted on the State list, even as a subspecies.
Another possibility (suggested by the obvious white tips of the secondaries) is first cycle Olympic Gull. This hybrid taxon derived from GWGU and Western Gull (Larus occidentalis), a hybrid form that is very common in the Pacific Northwest, has never been reported in Colorado to my knowledge, but presumably would be more likely than the SBGU x Vega combination. The strongest argument against Olympic Gull is the obvious barring on the scapulars, which unfortunately does not show up on the photos, which were taken in dim light at a great distance.
Opinions?
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All images in this gallery were taken by Thomas Heinrich about an hour after the initial, close-range observation. Thanks, Thomas.
