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Monday, December 5, 2011

The joy of long-distance Thayer's Gull identfication

One of my easiest remaining Kent Co. ticks is Thayer's Gull. Problem is, the only location in our lovely county which has a gull concentration is the Kent Co. landfill. And the viewing at this location is about as inopportune as it gets for gull watchers. The closest vantage is about .35 miles from the dump site, or 567 m (~5.67 football fields):Thayer's Gull identification is a quagmire in and of itself, so trying to do it at this distance is doubly troublesome. Some basic background: Thayer's and Iceland Gulls clearly constitute a clade, but instead of fitting nicely into 2 bimodal categories which we can easily separate (as suggested by the names), they form a cline of variation from black-primaried birds (=Thayer's) in their western arctic breeding areas to white-primaried birds in the European arctic (=nominate Iceland [glaucoides]). The 'in between' birds in the eastern N. American arctic, are grayish-primaried and are called "Kumlien's" Iceland Gull. The challenge with this group is deciding which phenotypes to classify as pure Thayer's, which as pure Kumlien's Iceland, and which to classify as unknown. Some of the birds between Thayer's and Kumlien's Iceland are even referred to as hybrids/intergrades, though on what basis (and what such a moniker actually connotes: if the bird looked exactly like both its parents how is that a hybrid/intergrade?!?) I don't claim to know. So to get to my bird from today let's start here: a plate comparing the wingtip pattern of large adult gulls, from "Gulls of North America, Europe, and Asia" by Olsen and Larsson (2003). Birds 13-16 are given to show the range of variation of acceptable Thayer's from darkest above to palest below, while bird 17 defies identification (would genetics show this bird to classify easily as one or the other when there's a cline of variation? I personally doubt it, but am not a geneticist so someone please relieve me of my ignorance), and plate 18 shows the extreme dark example of Kumlien's Iceland:
Long story short: I observed 2 adult kumlieni/thayeri today that were close to bird 17, but never could get an extended study of either bird, nor photos. But I did attempt to get video. I thought I had failed to capture my target, but in going through the video clip , I was shocked to see that the bird actually did unwittingly enter my screen, totally unbeknownst to me at the time (watch the upper left corner between 1:32 and 1:46). This bird, seen well, was darker gray above than Herrings, with dark eye and bright pink legs, as well as diffuse neck and head streaking. But n order to study the wingtip pattern of this bird I have created the following screencaps from the original clip, which are higher resolution that what you'll get on the youtube video (analysis to follow):

First off, the bird clearly applies to either bird 16 or bird 17 in Olsen and Larsson. How to decipher which? Well, the book doesn't really say in concrete terms. The caption to bird 16 reads: "Outer wing appears streaked blackish. Note black extension on outer webs of p9-p10, emphasizing white mirrors. Shows blackish markings as far as p5. Upperwing darker gray than in kumlieni." The caption for bird 17 says: "This example could represent minimal dark streaking in thayeri, with no dark markings on p5, or maximal dark streaking in kumlieni." How's that for hedging! Let's analyze this bird's outer primaries in more detail. I went into Photoshop and labelled each primary tip from the 2 best screencaps: It does appear that my bird lacks any markings on p5 (assuming I've labelled the feathers correctly), and that it lacks the black terminal markings on p10 shown on bird 16. The outer web of p9 pretty clearly lacks black in the area of the mirror, but so do both birds 16 and 17. The only other field mark I can garner from the plate would be the color of the pigmentation on these feathers: blacker on bird16 and slate gray on bird 17. I am not sure the answer to this one (nor the value it has as a definitive field mark!).

So, in the end, as so often happens, I think this is a bird we need to let go, as much as I would like to have a Thayer's Gull for Kent County. I left this and the other adult like it as Thayer's/Iceland Gulls on my eBird checklist. Birds like this may or may not "qualify," even with genetic data, as either species. But even if it did, we'd be hard-pressed to do so when its phenotype is so close to bird 17. Or perhaps we humans would do better to classify all birds in this clade as Thayland Gulls or Iceyeri Gulls? Alternative opinions here are welcome, of course (comment box below).

Fortunately for me, with upwards of 1,200 gulls present of late, as well as the influx of Thayer's in the western Great Lakes: I think the chances for finding a bird more like birds 13-15 (or heck, even a 1st or 2nd cycle bird) will be pretty good in the coming weeks. Let's hope that any such bird poses as well as this one did!


Cody said...


Thanks for the post -- I was actually thinking recently that I need to brush up on my Thayer's ID.

As far as your genetics question goes, the efficacy of genetic analysis in identifying confusing birds to species depends on what is actually occurring with these gulls. For instance, if the two phenotypes we call Thayer's and Iceland are simply the two "ends" of a cline, then introgression of alleles will muddle up analysis of individuals falling between the two extreme ends of the spectrum. As you mention, this seems to be the case, and is suggestive that these are not actually two distinct species. The BNA account for both species is pretty long and dry, but it has some interesting evidence to support this as well.

Of course, all bets are off when it comes to species limits in gulls -- a systemicist's worst nightmare!

RAZL DASL said...

Where's the post about the 1st cycle bird?