Tuesday, February 18, 2014

Often Sought, Often Misidentified: The Thayer's Gull




Adult Thayer's and a Mew Gull. Something to remember when attempting to ID adult gulls is to keep in mind how lighting, posture and digital photography can affect mantle color; note how pale the scapulars look on the Thayer's compared to the the rest of the wing. Miller-Knox Regional Shoreline, Richmond, CA.

Nothing brings more fear and loathing to the North American birder than gulls. True, birds like storm-petrels and Empidonax flycatchers can be vexing, but they just don't have the amount of staggering variation that "pure" gulls display in their many plumages, let alone the dizzying ensemble of hybrids that are out there.

One of the birds I am most often asked about for ID assistance (and see people constantly struggle with elsewhere) is Thayer's Gull. It is much sought-after by visiting birders, and even the birders that live here struggle with picking them out. Many seek this species, and many misidentify other gulls in the process. Here in the bay area, they are fairly common in winter, and occasionally can be found in large concentrations. So rather than flog the dead horse of Kumlien's vs. Iceland vs. Thayer's, I'm just going to throw up a bunch of pictures of Thayer's and what they are most often confused with here; Herring, Glaucous-winged X Western Gull, and Glaucous-winged X Herring hybrids.

This post isn't going to rewrite the book on Thayer's and I'm not into completely recycling what can be found in field guides, but hopefully this will be helpful/educational to someone down the line.



Same Thayer's, different Mew. This is a classic individual...long-winged, dark-eyed, round-headed, moderate markings on the head and breast. The bill is short and small with a indistinct gonydeal angle.



Another typical bird, with a slightly longer bill, which one would expect in a male. Gulls are sexually dimorphic, which explains a lot of the variation in size and structure within species. This photo also illustrates the pink-purple orbital ring, and most importantly, the inside of the folded primaries. Note the contrast between the whitish undersides of the primaries on the left wing and the standard black and white pattern on the upperside of the primaries of the right wing. This is a very useful field mark for identifying adult Thayer's. Photographed at Lake Merritt, Oakland, CA.



A heavily-marked bird. Even just that sliver of primaries visible from the right wing goes a long way to confirm this bird's identity as a Thayer's. Photographed at Golden Gate Park, San Francisco, CA.



Thayer's on the right, with a California Gull for comparison. Miller-Knox Regional Shoreline.



Light-eyed Thayer's Gulls are not uncommon in the bay area; use the underwing pattern and orbital color to tell them apart from Herring Gulls. Herrings are usually larger, but there is considerable size overlap. Golden Gate Park, San Francisco, CA.


This is the same bird as above. Although "string-of-pearls" is a phrase birders usually associate with Slaty-backed Gull, it's also what you want to see on the open wing of Thayer's Gulls.


Gulling would be a lot easier if you could get every bird to do this pose. The underside of the primary on p10 is a little dark (left wing) but I don't think that is unusual. Miller-Knox Regional Shoreline.



Here's the upperwing pattern of the same Lake Merritt Thayer's from above, for variety's sake.

I don't have much in the way of third cycle birds, so on to second cycles. As with younger birds, key in on the contrast between the inner and outer webs of the outer primaries to differentiate these from Herring Gulls. This bird was photographed at Golden Gate Park.


Same bird as above. Note the small, mellow bill and round head, creating a big-eyed look. This bird has slightly paler primaries than what I'm used to for second-cycle.



This isn't a particularly educational photo, I just like it a lot. Second-cycle Thayer's Gull, Golden Gate Park (different individual than previous photos).


Second-cycle. Marin Civic Center, CA


First-cycle with a lot of pink coming in on the bill. This is not unusual to see by mid-winter, although this bird is showing more than most. Photographed in Golden Gate Park.



Another first-cycle. Photographed in Golden Gate Park.


Same bird as above. This is typically what primaries of first-cycle Thayer's Gulls look like in mid-winter; long, dark brown with pale edges.



Another standard-issue first-cycle. Miller-Knox Regional Shoreline.




Similar to the above bird, but with more pink coming in on the bill. Miller-Knox Regional Shoreline.



Here is a very fresh, relatively dark bird photographed in mid-October, which is when Thayer's start arriving in the region. In fact, when I first saw the bird sitting on the water I thought it was a Herring because of how dark the primaries were. Note the pale inner primary panel and pale inner webs to the outer primaries, which separates this bird from Herring. This bird was far offshore, so those going on late-season pelagic trips should be on the lookout for Thayer's and other interesting gulls in the chum line. Photographed off Bodega Bay, CA.




Remember that dead horse I talked about? I guess I'll give it a kick after all. In the bay area, in late winter it is not unusual to see very worn and bleached Thayer's Gulls that look disturbingly whitish. Don't be fooled by these Kumlien's imposters. Photographed at Golden Gate Park.



Unlike the above bird, this one has feathers in good condition and is not heavily bleached. This creature has much more white in the coverts and flight feathers than the typical Thayer's. Could it be a Kumlien's? Maybe. Would birders in the eastern U.S. call it  Kumlien's? Maybe. Would California's Bird Police accept it as a Kumlien's? Nope. Do I know what it is? Not really, but I like it. Lake Merced, San Francisco, CA.


First-cycle Herring Gull. This individual is structurally very similar to Thayer's (many are not), but has a whitish head, extensive pink in the bill, and black primaries. Photographed in Golden Gate Park.




This Herring Gullish monstrosity bears more than a mild resemblance to first-cycle Vega Gull (note the white uppertail...more photos are available), but has a distinctly Thayer's Gull pattern to the outer primaries, and some bizarre mottling going on in the pale inner primary panel that doesn't belong on anything that I know of (double click for a closer look). I don't know what this bird is, but it's worth mentioning that given the tendency for large white-headed gulls to interbreed, we just don't know if/how much Thayer's Gulls are interbreeding with other species, as their breeding grounds are largely inaccessible. Seriously though...any ideas? Miller-Knox Regional Shoreline.




Here's a typical Herring Gull. Note the yellow orbital ring and obvious black and white pattern on the underside of p10 on the left wing. Golden Gate Park.




Olympic Gulls (Glaucous-winged X Western) are common in the bay area in winter. This bird is superficially similar to Thayer's in plumage but not in structure; it has short wings and a big bill that could give a smaller bird a brutal pummeling. Note the back pattern is somewhat sloppy and there are some new, dark gray feathers molting in on the mantle. Although this bird is pretty typical for a first-cycle Olympic Gull, its important to keep in mind that hybrids are incredibly variable. Olympic Gulls tend to be almost always larger and bulkier than Thayer's though (as there parent species are), so are not quite as challenging as Glaucous-winged X Herring, which to me show much more range in size. Miller-Knox Regional Shoreline.



Here is a putative Glaucous-winged X Herring hybrid. They are excellent impersonators of Thayer's Gulls, and can come in many shapes and sizes. On this one, note the heavier bill and indistinct patterning on the back; Thayer's are usually more boldly marked with primaries a tad darker, although with wear and bleaching that may not be the case. Golden Gate Park.



Another Glaucous-winged X Herring type (I don't think first-cycle GWGU x WEGU would get so much pink in the bill), perhaps resembling Thayer's more in plumage than the above bird, but with an even heavier bill and a small-eyed look. The tertials on this one appear very long compared to the length of the primaries, which is not something I associate with Thayer's. Miller-Knox Regional Shoreline.




An attractive Glaucous-winged X Herring adult. It could be confused with a pale-eyed Thayer's, but the gray primaries give away its hybrid vigor. Miller-Knox Regional Shoreline.




Here's a bird doing a good Thayer's impression...but look at the color of the underside of p10 on the right wing. It's too dark! When it took flight it confirmed an unlikely wing pattern for Thayer's Gull...I would guess this bird is also carrying a mix of Glaucous-winged and Herring genes, although in the field I thought the mantle seemed dark for that. I'm open to ideas. San Leandro Marina, San Leandro, CA.



Same bird as above. Note the extensive black in both the underside and upperside of the primaries (no string of pearls). The gonydeal angle looks very pronounced in this photo as well.


Thayer's have a reputation for being very small among those who don't actually see many of them, but in my experience that is not always the case. For example, take this light-mantled bird in front of these Western Gulls. I strongly suspect it is a big male Thayer's, but it could be a dark-primaried Glaucous-winged X Herring hybrid (or backcross) like the previous bird, as I didn't get a look at the wing pattern. Oyster Bay Regional Shoreline, San Leandro, CA.

Phew. That's a lot of gulls. If you would care to argue about any of the IDs I made in these photos, feel free to leave a comment. I'm ALWAYS willing to learn more about gulls, and for most of these individuals I have additional photos. At any rate, I hope you found this post educational, or at the very least, accurate. Gulling is a bitch.

9 comments:

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    1. No problem....I had to do something with all these fucking gull pictures.

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  2. TL;DR, but I assume the upshot is that there's no such thing as Thayer's Gull. Amirite?

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    1. You lazy bastard! To answer your abbreviated question, no. Some people think Iceland and Thayer's are essentially the same species, but despite that opinion being around for years no one has produced any solid evidence to support the claim. At any rate, Thayer's are certainly distinct from all the other large west coast gulls, which hybridize much more prolifically.

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    2. Haha. I actually did read and appreciate much of it. But I don't think these subtle IDs will ever stop driving me nuts. I'd be happy with "Big Gull," "Medium Gull," and "Small Gull." I just do not have what it takes to scale the heights of the GBRS. Anyway thanks for the detailed lesson, however lost it may be on clods like me.

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    3. The rarefied air I breathe as Number 7 is really worth trying...but the pain of gull ID is not something I ever expect to be numb to.

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  3. This is great and everything, but it does make my head want to explode. Might as well throw some logarithms and space/time continuoum stuff in here as well.

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  4. I would have probably called this one...

    http://2.bp.blogspot.com/-RTaZGSnnV04/UwK2yO4kJDI/AAAAAAAAQKg/7HeSI5-zWD8/s1600/gwguxwegu1.jpg

    ...just a Glaucous-winged. Wing tips seem to be just as pale as the rest of the bird.

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    1. To me the primaries and tertials are substantially darker. The wings are not showing the fine/faint patterning I would expect in GWGU; it looks much closer to what I would expect in WEGU, albeit paler.

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