Cameron et al.,



I have no intention of disparaging any of my colleagues birding talents,
least of all Cameron and Charlie. However, it seemed to me that Cameron's
post and Charlie before suggesting that Slaty-backed Gulls could be
identified at a glance by structure was misleading. First, of all, as Dennis
has noted, Slaty-backed Gulls are part of that perplexing suite of large
gulls that are largely allopatric in breeding range but which freely
interbreed where their ranges overlap, a group including in addition to
Slaty-backed, Vega and American Herring Gulls, Glaucous Gulls,
Glaucous-winged, and Western Gulls, at least (but apparently not including
the Thayer's-Iceland complex and California Gulls). They are all very
closely related genetically and therefore we might expect them to lack such
structural distinctiveness as might be characteristic of more genetically
isolated species.



I believe the identification of Slaty-backed Gulls does not and cannot begin
with "structure." On the contrary, one begins by picking out a dark-mantled
gull from a group of large gulls (assuming it is winter in the Pacific
Northwest toward the coast). One may assume, other things being equal, that
you're looking at a Western or a Slaty-backed. One next checks the head to
see if it shows the typical winter adult Western pattern of nearly
immaculate white with a small dark eye or the dusky nape smudges and the
football-shaped smudge about the pale staring eye of the typical winter
adult Slaty-backed. If so, you can already be 99% certain you're looking at
a Slaty-backed Gull. To this point you haven't had time or opportunity to
notice whether it is pot-bellied, long-necked, small-headed, or short legged
(which, according to the tarsi measurements in Olsen and Larsson is not
characteristic of Slaty-backed). Of course, at this point one will certainly
want to run for a camera and then to check the proportions of the bill, pink
tone of the legs (if it has yellow legs, think again), and hope for a
spread-winged shot to see the paler underwing and, perhaps, the
"string-of-pearls" so often talked about with regard to Slaty-backed Gulls.
I agree with Cameron that that field mark is overrated, as Vega Herrings and
even Westerns can show it, at least to some degree and it is not easy to see
clearly.



For sub-adults and immatures, I think one would need to look at other
details, as well as structure, but one would, of course, begin by assessing
the birds' approximate age.



The point is, gull identification is neither mystical nor easy. I say
"mystical" because "structural" qualities are firstly nearly impossible to
measure, secondly very difficult to describe, and thirdly highly variable on
even individual birds as they shift their postures. Thus claims about the
distinctiveness of structure are, in my view, scientifically suspect. If one
can learn to recognize such "gestalt" qualities, more power to you. It will
perhaps help you pick out unusual species. But it can never substitute for a
careful, point-for-point analysis of "objective" properties, with due
humility as to the range of variability within species, most notably our
large gulls.



Gene Hunn

18476 47th Pl NE

Lake Forest Park, WA

enhunn323 at comcast.net







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