Update, Oct 1, 2015: Then there are individuals like this Mew Gull, also seen in the Bay area, which seems more leucistic than bleached. Or maybe just the primaries are bleached? Who knows.
During our July trip to the Bay Area, my girlfriend Lesley and I encountered two rather strange pale gulls. The first was at Zmudowski State Beach, and the other on the Golden Gate Beachfront. Acknowledging that I do not have much experience with confusing gulls, I originally thought both birds were Glaucous Gulls, based on my Alaskan experience with this species. But further investigation revealed these are actually a bleached juvenile Glaucous-winged Gulls.
Scanning the internet shows that bleached Glaucous-winged Gulls are not that uncommon, but their IDs do seem to pose a challenge. Pacific NW Birder discussed a spectacularly pale Glaucous-winged Gull he photographed at Newport, Oregon in 2011. Very similar-looking birds were also photographed at Point Reyes in 2009, and Santa Barbara in 2005 (both these latter posts never volunteered a definitive id).
Plumage characteristics has of course been the downfall of many a reputation, so one would rather choose structural features when facing identification challenges. We’re realistically assuming these two gulls are one of the larger pale gulls (Glaucous, Glaucous-winged, Iceland). In both these photos the gulls have a heavy dark, strongly hooked bills with swollen gonys, clearly not right for Glaucous Gull, that would have a pink or bi-colored bill, but more importantly lack the swollen gonys. Based on the all-dark bill, a first-year Iceland Gull might be a better guess. However, Iceland Gulls have a long primary projection, a petite body and a rather small dainty bill. None of these features are present on the fairly large birds I photographed in California. So, through elimination we’re left with Glaucous-winged Gull, further confirmed by the pink legs and feet, and pink gape with paleness at the base of the lower mandible on both the California birds.
Looking through all these pictures it strikes me that all these pale Glaucous-winged Gulls are first-year birds. Could it be that juvenile birds’ first batch of unprotected feathers of are more prone to bleaching when exposed to the elements, compared to subsequent feathers having more protection during molting? It would be interesting to know which circumstances would bring this about.
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