I was looking through my unread Tweeters digests and came across a
message I couldn't pass up. Hope you're not too sick of this thread
already....

>>Gene Hunn wrote:
>>
>>"Re. light-eyes on Thayer's. It depends on what one means by "light"
>>perhaps. My impression of ("typical"?) Herring Gull adults is that their
>>irises are almost white, color of white gold perhaps, really glaring,
>>while the ground color of Thayer's, and the dominant color when the brown
>>flecking is minimal is more a rich yellow. Do others agree?"
>

and Dennis Paulson responded:

>Don't know if I do. I think they overlap. I have a bunch of closeup
>photos of Herring Gull heads from the Great Lakes (birds that were
>anesthetized to be banded, weighed, and sexed, also photographed) that show
>some birds, by the way, with a surprising amount of brown flecking in the
>iris. They look pale yellow, not white, as--to me--do the palest Thayer's
>eyes.

I spent a few hours today looking at Thayer's Gull's eyes myself. (I
think the total number of hours I've logged scrutinizing the iris color
of gulls is well into the hundreds by now--I don't exactly know what that
says about me.) Anyway, what I saw today was consistent with what I
remembered, and that is that there does indeed seem to be a difference,
in general, between the iris color of Herring and pale-eyed Thayer's.
First of all, pale-eyed Thayer's are a definite minority among Thayer's.
Certainly less than 5% of the birds we get on Sauvie Island could be
defended as being pale-eyed. Of those, the marginal ones look pale-eyed
only in bright light, while even in dim light, Herring Gulls irides
always look pale yellow. The majority of the rest have more brown flecks
than any Herring I've seen around here.

I'm sure Dennis is correct in saying that there is overlap in eye color
between Herring and Thayer's. He has data to prove it. But I am
equally sure that this is very uncommon, at least in our part of the
world. I've seen perhaps two or three Thayer's here ever that had very
pale yellow, off-white eyes. I think they were as light eyed as any
Herring Gull's I've seen. (One even lighter.) But whatever population of
Thayer's it is, (Kumlien's perhaps?) that supplies these birds to us, it
doesn't do so very often. (I know Dennis and Al will jump all over that
bit about population. Go ahead, I'm wearing my asbestos suit!) My
point in all this is that Gene was correct in that there are differences
in iris color that can be used in the field.

Since I'm already skating on thin ice, I might as well add that I think
there might also be a correlation between iris color and bill size in
Thayer's Gulls. The ones with extremely pale eyes, and again there
aren't many, almost always (always?) seem to have smaller than average
bills. As an example, see the one Dennis photographed in _Birds of
Alaska_. The two or three almost white-eyed Thayer's I referred to above
had tiny bills hardly larger than a Mew Gull might have. I'll have to go
back and reread Neal Smith's paper on the evolution of Thayer's/Iceland
Gulls to see what he had to say about bill size differences. I can't
recall. (I know some people think Smith's data was cooked, but I think
his general conclusions may still have a lot of merit.)

By the way, those light-eyed birds I referred to didn't have
particularly light primary tips. Perhaps primary darkness is not what
we should be using to differentiate Kumlien's from Thayer's? Maybe they
really *are* different species and we've just been looking at the wrong
physical characteristics? (Oh boy, now I'm really in for it!) But
seriously, is there any hard evidence that there *isn't* assortative
mating by eye-color among some populations of Thayer's?

One last comment, and then I'll shut up. After Dennis mentioned to me a
year ago about the brown-flecking in the eyes of Herring Gulls, I was
fortunate enough to get the opportunity to look through some Herring
Gull's wintering in Ohio. I'd never been able to see any brown flecking
in Herring Gull's eyes in all my hours of Northwest gull watching. But,
lo and behold, a couple of those Great Lakes birds did appear to have
tiny, fine-grained brown flecks in their eyes. Some of them also seemed
to have somewhat bigger bills with more noticeable gony angles than do any
of our Northwest Herring Gulls. I wonder if there are population
differences that we don't know about in wintering Herring Gulls as well?

Always in rampant speculation,

Skip

--
Skip_Russell at intersolv.com Voice: 503-690-4214
Beaverton, Oregon FAX: 503-629-9637