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Sunday, March 11, 2012

possible dark morph Ross's Goose- an ID quandary

This post is going to be shorter and less extensive than a full analysis, mainly because I do not have the necessary days to do the research. But I think the topic is important enough to attempt to make some progress in lieu of a deeper literature and internet photo review.

The basics: the very rare 'blue morph' Ross's Goose (ROGO) is, for me, a very mysterious critter. I have never seen one in life (maybe 2 total so far) that I was sure wasn't a hybrid or backcross ROGO X Snow Goose (SNGO), and I have never felt that I had a firm grasp on even the bird's very existence. Many of the putative photos I have seen from other states appeared to be phenotypically intermediate, and there is a suggestion that no 'pure' blue morph Ross's Goose exists at all. The Crossley Guide says "blue morph [ROGO] is extremely rare and perhaps a hybrid with SNGO where the blue-morph gene is dominant." National Geographic says "the origin of this blue morph is controversial and is though to be due either to introgression with blue [SNGO] or a recurrent mutation of genes controlling feather color." The source of such sentiments seems to be a handful of papers referenced in the Pyle Guide II. [Pyle, incidentally says "Dark-morph Ross's Geese are very rare and may represent hybrids or back-crosses with dark-morph Snow Geese..."]. Here is one of the citations given in support of this statement:


and the other 3 (which I cannot currently locate online) are:

Williamson, M.H. 1957. Ploymorphism in Ross's Geese (Anser rossii) and the detection of genetic dominance from field data. Ibis 99:516-518.

F. Cooke and J.P. Rider. 1979. The genetics of polymorphism in the Ross's Goose. Evolution 25:483-490.

Kaufman, K., J. Witzeman, and E. Cook. 1979. Pinning down the blue Ross' Goose. Continental Birdlife 1:112-115.

I am not going to go any further into the content of these papers (much of it is well beyond my expertise), other than to say that this apparently well-supported hypothesis (that blue morph Ross's may not exist, since the dominant allele coding for blue morph is strictly a SNGO allele; and that correspondingly any blue morph goose must have some SNGO in its recent ancestry) has long given me pause at accepting any claim of this bird. So it was with extreme interest that I heard of an apparently good candidate in SW lower Michigan (!) Friday evening. I was able to visit the site on March 10 and to obtain fairly close up digiscoped video of the bird while studying it for 1 hour in the late morning. The footage was taken using a Nikon Coolpix p5100 through my Kowa Prominar 884, with Kowa digiscoping adapter and window mount. I did not attempt any still shots. Here are the raw videos (be sure to select 480p quality when viewing):
and here are the best screen captures (please see my discussion of this bird's identity at the bottom of this post):
























A few things. First, this bird was with an unprecedented number (for Michigan) of white geese (~152-214 individuals) with somewhere between 26 and 69 pure Ross's Geese , a Michigan high count, at least 30 individuals which are hybrids/backcrosses, and at least 76 pure Snow Geese. Sorting through this mess of morphs, age classes, and intermediate phenotypes presented its own challenges, but here is my impression of the bird's actual traits:

1) Body size. My impression was that this bird was perhaps 5-10% more bulky than the smallest, apparently pure ROGO which were present. But I honestly don't know that this makes the bird too large for a pure ROGO, since there is sexual dimorphism and other allowable size variation within the species. It seemed small enough for what I consider a pure ROGO, but not as small as some individual ROGO.

2) Bill base/facial feathering junction verticality: I tried very hard to discern the exact pattern of this trait, and left not completely sure of its true expression. Most of the time this juncture appeared perfectly or nearly perfectly vertical to me, but at times (visible in some, but not other, captures above) there seemed to be a slight anterior bulging of the facial feathering, or a slight posterior incursion of the superior bill lobe onto the bird's face. I honestly do not know which view is correct, but most of the time the bird seemed to be within the normal range of ROGO to my eye.

3) Bill size and shape: the bird's bill size and shape seemed rather good for pure ROGO most of the time. It was small, triangular, and did not form a continuation of the outline of the bird's forehead like a Canvasback or Ammodramus sparrow (something which often makes me think a bird is intermediate), instead having a 'step down' at the forehead separating the crown from the culmen. I hope I am describing this trait amply. I have seen a dazzling array of expressions of this trait and honestly am not sure how 'flat foreheaded' a bird can be while still being a 'pure' ROGO. But this bird seemed within range of what I consider a pure ROGO, but with less of a 'step down' than some pure ROGO I have seen in person and in print.

4) Tomial 'grin' patch: Although the grin patch looks unnervingly large in some of these captures, I believe that the bird's bill was open in all of these instances. One may be able to confirm this by watching the videos. When the bird's bill was shut fully, as is visible in many of the screencaptures, the patch was very small and of uniform width throughout the bill's length. I do not believe it to be out of the range of normal ROGO.

5) Plumage: The bird's plumage coloration overall seemed consistent with my understanding of a 'typical' blue morph Ross's Goose: black not brown body coloration, white (not gray) wing coverts, white belly, white face, black nape and neck sides, etc. It is an adult based on the fully pink bill and clean white face and belly, I believe (someone please correct me if this is wrong!). But 2 of the bird's traits gave me some pause, not necessarily because I 'knew' they were out of range, but because I wanted some clarification of whether the literature allows for it in a pure ROGO. The traits are a) the lack of extension of the black on the nape all the way onto the bird's crown, and b) the irregular white patch amongst the black on the bird's flanks rather than the more uniform black flank area depicted in Sibley and Nat'l Geographic, etc. I would really appreciate feedback as to whether these 2 traits are acceptable for a blue morph Ross's Goose or if they might be indicative of SNGO introgression. I would also appreciate knowing whether I have missed any other traits which are 'off' for this putative identification.

I think this is enough to get the discussion started. In summary, the bird seems like a strong candidate for a 'pure' blue morph Ross's Goose, to my eye, with a couple possible caveats. Please reply in the comments box below or to my email address caleb.putnam (at) gmail.com

1 comment:

Jerry said...

Brilliant post, Caleb!