Andrew,

I think the situation with Western and Glaucous-winged Gulls is rather more
complicated than simple hybridization of "pure" forms. It is my
understanding that most of these large gulls nesting on the Washington coast
nest in mixed colonies and that in mate selection there is a tendency for
like birds to pair up, which includes hybrids, but within the limits of
available mates, presumably. As there is no particular selective advantage
associated with species "purity" one will find a range of crosses, back
crosses, and double crosses (to invent a non-existent category). I rate the
large Glaucous-winged-Western type Gulls I see on a ten-point scale, 10 for
a "pure" Western, one for a "pure" Glaucous-winged.

These "Olympic Gulls" will typically exhibit a mantle-color (in adults or
near adults) intermediate between "pure" northern Western Gulls (which are,
as you note, considerably paler than southern Westerns, but still perhaps
normally a shade darker than a California, for example) and "pure"
Glaucous-winged Gulls (which may be slightly darker than Glaucous-winged
Gulls that breed on the Aleutians). They will also tend to show dark gray
wing tips, somewhat indistinctly demarcated along the lines of the blackish
wing-tips of a Western but distinctly paler than the Western. I don't know
about "streaked" wingtips. The tone of the primary tips can be hard to
assess until the bird flies, however, and what may appear black on a
standing bird may look distinctly gray in flight. Another useful feature, on
winter adults, is the extent of streaking or smudging on the head, as
Western's tend to have nearly immaculate white heads in winter, quite unlike
Glaucous-winged. Hybrids (or intergrades if you prefer) should be
intermediate in this regard as well. As for immatures, it may boil down to
overall plumage tone, paler to darker, with particular focus on the
primaries.

The two taxa have not been lumped, I understand, because the zone of
intergradation or hybridization appears to have been stable for at least the
past century, suggesting that there are some forces restraining total
mixing.

I'm sure there are biologists out there who could offer a more nuanced
account.

Gene Hunn
18476 47th Pl NE
Lake Forest Park, WA 98155
enhunn323 at comcast.net

----- Original Message -----
From: "Andrew Reding" <aareding at gmail.com>
To: <tweeters at u.washington.edu>
Sent: Wednesday, September 26, 2007 10:52 AM
Subject: [Tweeters] thoughts on those confusing large gulls


> Thanks for the responses to my query about the large gull. Unlike the
> previous query which elicited unanimous identification of a Pacific loon
> in breeding plumage, responses in this case were divided, with three
> saying western, two saying hybrid, and one saying glaucous-winged. The
> latter is in fact impossible given the large black triangle at the tip of
> the wing.
>
> Where, however, does one draw the line between westerns and hybrids? To
> the extent that one can rely on the Sibley field guide illustrations, a
> solid dark triangle at the tip of the wing (as opposed to a streaked one)
> would be definitive of a pure (or almost pure) western. The other
> seemingly clear line of demarcation is an orange-yellow (as opposed to
> pink) orbital ring. To inspect the orbital ring I zoom in close with my
> telephoto and examine it from digital photographs.
>
> Perhaps from an overabundance of caution, I have noticed a tendency on the
> part of many local birders to automatically label any darker gull as a
> glaucous-winged x western hybrid, forgetting that it is only possible to
> have so many hybrids if there are a decent number of pure strain westerns
> around. Otherwise there would be no way to hybridize!
>
> I also have doubts about using depth of mantle color as a criterion. There
> is a clinal variation among westerns in which the degree of gray declines
> as one goes north. So a lesser amount of gray does not necessarily mean it
> is not a western.
>
> The other possibility is that western and glaucous-winged are really just
> a single species with continuous clinal variation. There is always
> something suspicious in biology about having too many hybrids.
>
> While on the subject of taxonomy, isn't it peculiar that we are now told
> to lump the easily distinguishable red and yellow-shafted flickers as a
> single species, yet still told to distinguish the indistinguishable
> northwestern crows from common crows?
>
> Andrew
>
> Here are two mystery gulls:
> http://picasaweb.google.com/aareding/Wildlife
>
> One of them has helpfully raised his wings. From the Sibley guide
> illustrations, i am guessing western gull ... am i right?
>
> Please click on "view largest photo" or "download largest photo" to get
> closer to original size.
>
> Thanks,
>
> Andrew Reding
> Port Townsend
> aareding at gmail dot com
>
>
>
>
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