[Birdtalk] Comments on Pat's Photos, Please

[Mark Stackhouse]

Curse you Kris!

Not because I chased your gull (wish I had time for some birding), but 
because of the headache you've given me thinking about this i.d. 
problem.

I'm beginning to come to the opinion of many ornithologists who believe 
that Thayer's Gull is only a subspecies of Iceland Gull. Even in the 
adults there is overlap in every character between nominate Iceland, 
Kumlien"s, and Thayer's. Many of the features described in the field 
guides as being useful for identifying these species are based upon 
photos and specimens that are not of known parentage (that is of 
unknown origin with regards to the breeding area - the only "certain" 
way to tell these forms apart). Therefore, the identifications of the 
specimens are based upon the best considered judgments of the 
observers/collectors, resulting in a circularity in the reasoning on  
how to identify these species. The few studies that have been made of 
birds of certain origin have not turned up any characteristics or suite 
of characteristics that are reliable for identifying these "species."  
For example, for years it was thought that all Thayer's Gulls had dark 
irises, and both Kumlien's and nominate (L. g. glaucoides) Iceland had 
pale irises. However, studies of birds on the breeding grounds showed 
Thayer's with pale irises and Kumlien's with dark irises, and even some 
glaucoides with heavy enough speckling in the iris that it would appear 
darker in the field. The DNA differences are miniscule to non-existent 
- but then they're not great between any of the large, white-headed 
gulls. My head hurts.

Here in the west we torture ourselves to find a "good" Iceland Gull; in 
the east, they do the same to find a "good" Thayer's.

With that rant out of the way, your bird looks pretty good to me for a 
non-Thayer's Iceland, though it's difficult to see everything that 
might be useful from the photos, even though they're a good result from 
a distant scope view. The mantle appears pale, and the primaries 
patterned (indicating probably a Kumlien's, and not glaucoides), but 
there doesn't seem to be much dark in the primaries, which you would 
expect for most Thayer's. The dark eye you mentioned (not visible in 
the photos) would be better for Thayer's, but is not definitive. I 
can't tell about wing length from the photos, but longer is better for 
Iceland. I agree with Tim that the bill and head shape look better for 
Thayer's (larger, heavier bill and flatter head), but I just read that 
these widely-used field marks are probably indicative of differences 
between sexes, rather than species - and if this is true this bird 
would be a male.

Hopefully this bird will show itself again, and give us better views 
and photos, so we can really torture ourselves.

Good birding!

Mark Stackhouse
mark at westwings.com
801-487-9453 (Salt Lake City, Utah, USA)
011-52-323-285-1243 (San Blas, Nayarit, Mexico)

On Feb 27, 2007, at 9:59 AM, Kristin Purdy wrote:

> I invite discussion on the identity of the possible Iceland Gull seen 
> yesterday at Farmington Bay. One analyst has scrubbed the photos that 
> Pat Jividen circulated on a birdnet post and suggests a Thayer's Gull 
> is a more likely ID for this bird. Please feel free to comment!
>  
> Should the conclusion be that the bird is a Thayer's, I apologize for 
> unnecessarily sounding the tweet alarm that sent folks to Farmington 
> Bay for a bird classified only as rare. At least I hope 
> everyone benefits from the discussion, especially, me!
>  
> Many thanks to Pat Jividen for capturing these shots. Shooting images 
> through a scope without benefit of an adapter is a Herculean task, in 
> my opinion, and I'm amazed that Pat was able to capture shots that 
> demonstrate several structural characters of the bird.
>  
> Kris_______________________________________________
> Birdtalk mailing list
> Birdtalk at utahbirds.org
> http://utahbirds.org/mailman/listinfo/birdtalk
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