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From: dmark@joey.cs.buffalo.edu (David Mark)
Newsgroups: rec.birds
Subject: Re: What is this bird??
Message-ID: <10772@eerie.acsu.Buffalo.EDU>
Date: 23 Sep 89 14:06:06 GMT
References: <3791@helios.ee.lbl.gov> <48214@oliveb.olivetti.com> <3192@nmtsun.nmt.edu>
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In article <3192@nmtsun.nmt.edu> john@nmtsun.nmt.edu (John Shipman) writes:
>
>While we're on the subject of nasty field identification
>problems, how about hybrids?  The Christmas Bird Counts
>from the northern Pacific coast in the last few years
>often list more Glaucous-winged x Western Gulls than
>either of the unhybridized species.  How are they
>calling this form?

A phenotypically pure glaucescens has no black whatsoever in the wings.  
Essentially all of the big gulls breeding on the British Columbia coast
on the "inside" of Vancouver Island, and north of Vancouver Island, are like
this.  But, on the "outside" of V.I., a small proportion have black or
very dark areas on the upper side of the primaries.  I went out to Cleland
Island off Tofino with Mike Sheppard, then of the Provincial Museum, to band
gulls.  There were 3 phenotypically-pure "occidentalis" types among the
several thousand "glaucescens", two "occidentalis" paired to eachother and not
breedings, the other paired to a glaucescens.  I recall that I estimated about
1-2 % of the gulls there appeared to be hybrids.  Around Westport, Washington,
in summer, it is very hard to find "pure" occidentalis *OR* glaucescens, with
most birds being intermediate.  Frankly, I don't understand why they have not
been lumped.  It is obvious that the GULLS don't care about the difference.
Hoffman, Wiens, and Scott (Auk 95:441-456, 1978) studied the hybridization
along the Washington Coast, and found that hybrid pairs had higher breeding
success than pure tryue-to-type pairs.

In color, adult "gloccidentalis" birds resemble thayeri.  Primaries are
dark above but not below, dark iris.  But the 'jizz' is quite different,
with thayeri adults being rather 'delicate', looking almost like large
Mew gulls, or at least like Californias, whereas 'gloccidentalis' are big
bulky birds with large long bills, flatter heads, etc.

Another hybrid that occurs around Vancouver BC in winter is HerringXGlaucous-w.
These also have a similar phenotype to the above hybrid and to thayeri.  They
come from Cook Inlet area, when interior Herring types spead from dump to 
dump out to the coast.  Once, I saw an adult, color-banded gull near
Vancouver, which had this dark-above, light-below pattern on the primaries.
I sent in the color pattern to the banding office and it turned out to have 
been banded as a chick in the Cook Inlet area.

Obviously, observers in southern California or the southwest, who perhaps
are unfamiliar with thayeri, should be cautious in identifying adult gulls
with the primary pattern mentioned above as Thayer's Gulls.

David Mark
dmark@cs.buffalo.edu