Saturday, 26 June 2010

Glaucous-winged Gull at Beddington Farmlands?

Photo by Garry Messenbird
Photo by Kevin Guest

CASE FOR: A worn first-summer female Glaucous-winged Gull: A relatively fine bill and herring gull like structure consistent with size variation of GWG. All dark tail (darkest part of bird), heavily barred upper and under-tail coverts, worn dark mantle/scaps and overall uniform bird with bright pink legs and all dark bill consistent with GWG. The eye also appears in some photos to be high in the head. The pale tips to the uniform (with the rest of the bird) primaries is a typical function of ware. Some similar birds are shown in the post below.
CASE FOR: A first summer 'west coast (Glaucous-winged Gull) hybrid': The fine bill and short secondaries are structural inconsistencies with GWG. The combination of these two atypical features is very unusual (1.4% of birds in a quick analysis). The herring gull like structure to the Beddington bird could indicate the presence of American Herring Gull genes suggesting a GWGxAMHG or even a GWGx(AMHGxGWG). There is an inconsistency of acceptable variation within GWG between published literature (scientifically approved) and the Internet (uncensored). There are unanswered matters concerning the full nature of the extensive hybrid swarms on the west coast of North America and identification criteria for various hybrids and inter-species variation of this complex has not been adequately documented. Is it therefore impossible to rule out a hybrid with certainty/high degree of confidence?


Darryl said...

Shurely shome mishtake - that second paragraph should be the case AGAINST :-)

Peter Alfrey said...

yesh it meant to say case for- i think it is a case for either a Glaucous Winged Gull or a case for a hybrid. In my mind the case is equally strong for both scenarios- a choice is required and either choice can be justified. There may be new factors to bring to the table but I cannot find a totally convincing argument either way (so far).

Johnny Allan said...

It is, of course, impossible to say with 100% accuracy if a bird is pure GWG or not, even if it is a 'classic'. Is there enough knowledge available to be able to present an objective picture of acceptable extralimital variations of this species or do they all have to be 'classic' ? Don't recall the 2007 bird having such a hefty bill as some. Sibley's comment in 'The North American Bird Guide', "some small-billed, close to Thayer's" is interesting.

Peter Alfrey said...

I think all gulls need to be classics or at 'worse' near classics to be assigned a specific name. Rather than try and squeeze indiviudal birds into boxes for the purpose of gaining a tick or for convenience I personally prefer to give the hybrid a name eg. Viking Gull, Nelson's Gull, Kumlein's Gull and then 'tick' that. I dont think the truth should be bent to fit a game (listing)- I would rather the game is adapted to fit the truth.
With the Beddington bird the jury is still out and hopefully more info will come to light to push it from an either/ or to one way or the other. May be will never now- then I will tick 'Impossible Gull'.