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11282Cook Inlet vs. Slaty-backed: a summary

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  • Tristan McKee
    Feb 9, 2014
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      Hi Calbirders,

      Regardless of its identification, the juvenile gull frequenting downtown Arcata this winter provides an unparalleled opportunity to ponder Slaty-backed characters up close, in direct comparison with Glaucous-winged and Herring gulls and their apparent hybrids. These hybrids are the source of the popular notion that first-cycle Slaty-backed Gulls simply cannot be identified here, in the absence of incoming slaty mantle feathers. However, the Arcata bird shows numerous traits that differ from both species and are not intermediate between them. I have also just uploaded a zoomed-in photo that shows an incoming bluish-slate-colored mantle feather (I am unsure if this is diagnostic, so I'm not placing much weight on it at this point):

      http://m.flickr.com/#/photos/101791769@N08/12394547043/

      Since development of identification criteria for this cycle has largely occurred on the internet and is widely scattered, I have summarized the results of my work this winter to distinguish Slaty-backed from "Cook Inlet" hybrids. A more detailed account will be published shortly, but I figured hey, it's February, and there is still time to put some of this into practice before the end of gull season.

      Two of the most helpful references for Slaty-backed ID are Moores (2005) and Zimmer (2000):

      http://www.birdskorea.org/Birds/Identification/ID_Notes/BK-ID-Slaty-backed-Gull.shtml

      http://books.google.com/books?id=TMtB8CG767EC&pg=PA163&dq=Slaty-backed+Gull&hl=en&sa=X&ei=ywD3UqyYDoneoAS_3IHwBQ&ved=0CDMQ6AEwAA

      Probably the best way to familiarize yourself with this extremely variable bird, without going to Asia, is to spend some time on the Japanese Gull-Site:

      http://www23.tok2.com/home/jgull/gullidentifi_.htm

      Also be sure to read Chris Gibbins' excellent blog on this topic:

      http://chrisgibbins-gullsbirds.blogspot.com/2010/02/first-winter-slaty-backed-gull-bit-of.html?m=1

      I highly recommend exploring the behavioral ecology work of Yutaka Watanuki on this species, which gives a bit of context for the phenotypic plasticity that has been at the root of our confusion as birders.

      Identifying birds in this cycle is not unlike identifying Accipiters. There are few cut-and-dried field marks, but with a little practice, we can learn to apply a whole suite of structural and plumage characters that make the identification of most individuals relatively clear. Admittedly, rampant hybridization forces us to let a higher percentage of gulls go, but that does not mean we should shy away from those birds displaying ten or twenty traits that add up to a reasonable conclusion.

      First, I should mention that the Arcata bird lacks deep pink legs. This is not uncommon on first-cycle Slaty-backed Gulls in Asia (see the Japanese Gull-Site).

      The following are 14 key traits of Slaty-backed (in no particular order) that differ from the Glaucous-winged/Herring continuum. Note that Slaty-backed is tremendously variable, and we should not expect all (any?) birds to have ALL the "classic Slaty-backed" traits. I have added links to photos of the Arcata bird that illustrate these concepts.

      1) Short wings. This is often used by the Japanese experts.

      http://m.flickr.com/#/photos/101791769@N08/12314683794/

      Note that this species also seems to have exceptionally flexible wings. The humerus is often extended significantly. In combination with folding the secondaries inward, this can give the wings an unexpectedly long, narrow look:

      http://m.flickr.com/#/photos/101791769@N08/12267289063/

      2) The primary panel looks short and narrow, both on the sitting bird and in flight (except, of course, when the hand is fully spread). I have discussed this in a little more detail in the captions of these three photos:

      http://m.flickr.com/#/photos/101791769@N08/12314407293/

      http://www.flickr.com/photos/101791769@N08/12314260615/

      http://www.flickr.com/photos/101791769@N08/12272532173/

      3) Short, stout bill with minimal gonydeal angle and slightly drooping appearance:

      http://www.flickr.com/photos/101791769@N08/12314409633/

      4) Contrasting dark mask through the eye and onto the auriculars. Herring tends to be plainer on the face, while Glaucous-winged can have a dark smudge through the eye and onto the cap, and, sometimes, onto the auriculars. In the "classic Slaty-backed look", this dark mask is set off by a whitish half-collar on the side of the neck and a whitish area around the bill, as well as the rather bold eye crescents.

      http://m.flickr.com/#/photos/101791769@N08/12295002084/

      5) Plain-based greater coverts (dark brown when fresh, white later). This and all similar species have variable greater covert patterning, however.

      http://m.flickr.com/#/photos/101791769@N08/12294718133/

      6) Distinctly decurved gape, which is shared with California but generally not with Glaucous-winged or Herring. I have seen Glaucous-winged Gulls that approached this look, however.

      http://m.flickr.com/#/photos/101791769@N08/12402310734/in/photostream/

      7) Long mid-secondaries. These look like a point or bulge when spread, much like a skua. This is not obvious when they are more folded.

      http://m.flickr.com/#/photos/101791769@N08/12255374275/

      8) Contrastingly white background to rump. Many Slaty-backed Gulls have uncontrasting darker rumps, especially in fresh plumage. A contrastingly white background, however, is also common and differs from the darker rumps of both Glaucous-winged and American Herring gulls.

      http://m.flickr.com/#/photos/101791769@N08/12279690664/

      9) Many birds have a rather blocky-looking head shape, with a concave slope to the forehead. Glaucous-winged tends to have a more evenly rounded forehead (convex), while Herring has a rather flat-looking forehead. Conceivably, a hybrid might combine these two looks into something like Slaty-backed.

      http://m.flickr.com/#/photos/101791769@N08/12402147224/

      10) On many birds, the inner primaries are darker than either Glaucous-winged or Herring. The "shadow string of pearls" effect can be quite striking.

      http://m.flickr.com/#/photos/101791769@N08/12402310734/

      11) When resting and often while walking, Slaty-backed exhibits a very forward-hunched, pot-bellied, chest-heavy posture.

      http://m.flickr.com/#/photos/101791769@N08/12255795685/

      12) The legs of Slaty-backed vary from pale fleshy pink to dark, vivid pink. The latter is fairly distinctive, but note that low light angles can make any gull's feet look quite vivid.

      13) The legs vary greatly in length but are often notably widely-spaced, and the gait has been described as waddling or goose-like. A particularly thick-legged look is also characteristic but not always evident.

      http://m.flickr.com/#/photos/101791769@N08/12267708256/

      14) When alert, the neck looks very long. Other gulls can certainly approximate this look, however.

      http://m.flickr.com/#/photos/101791769@N08/12279614836/

      It is important to note that not all of these differences would apply to a Vega x Glaucous-winged hybrid. Therefore, it is crucial to confirm that dark brown extends to the base of the tail (though occasional apparent Slaty-backeds do have white mottling across the entire tail base; such a bird would likely be much more difficult to establish as a vagrant).


      Investigation of this formerly very popular identification problem seems to have dropped off drastically since I was a kid. This seems to stem from frustration with all the appearances presented by the prevalent hybrids in this state. Nonetheless, working out these issues has very much been a community effort, and I hope that continues; I would love to hear more discussion and debate on this matter. I'd particularly like to thank Zachary Ormsby, John Sterling, Ken Burton, Alvaro Jaramillo, Steve Hampton, Peter Pyle, Jude Claire Power, David Fix, Osao and Michiaki Ujihara, Rob Fowler, Amar Ayyash, Paul Lehman, Steve Howell, Martin Meyers, and Steve Rottenborn for joining me in grappling with this problem over the course of the winter.

      Go find some county first Slaty-backed Gulls!

      Tristan McKee
      Arcata, CA
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