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Tuesday, March 8, 2011

Putative Ross's Goose

Received a call today from Randy Vandermolen (up and coming Kent Co. birder extraordinaire) that he had briefly viewed and photographed a small white goose in with 20 or so Canadas, which landed out of sight. After some deliberation, and based on his description, I decided there was a decent chance the bird was a Ross's candidate. Upon meeting Randy and looking at his photos from his SLR screen, I was convinced the bird was far too small for a Snow Goose. We found the bird around 3PM and snuck up on it, getting these photos:

As with all Ross's Geese, one must consider and eliminate all possibilities of Snow Goose X Ross's Goose hybrids and backcrosses in order to count such a bird. This one is no exception. My analysis is still underway, but currently goes about like this:

Pro Ross's traits:
1) Body size (not as small as some Ross's, but not too big to cause alarm as there is sexual dimorphism in this trait).
2) Blue base to bill, extending from bill base to nares
3) Lack of a grin patch on the tomium of the bill
4) Beak not too long or bulky overall

Indeterminate traits:
1) Verticality of the bill base/facial junction.

To me, this bird's ID hinges upon this latter trait. The classic Snow Goose trait here is a long, regular, sloping curvature to the bill base, especially where it meets the upper mandible. The classic Ross's trait is a perfectly vertical junction. This bird is somewhere in between, but I think it is much closer to the Ross's condition, and probably within the range of variation of that species. Specifically, if you take away the small incursions at the gape and the upper lobe of the bill (perhaps relicts of juvenile bare parts? [this bird is last year's young based on the retained juvenal gray crown and mantle feathers]), the feathering which meets the upper mandible is indeed vertical. So at this point I am leaning that this is a pure Ross's Goose. That said:

There is no universally agreed upon definition for where to draw the line between "pure Ross's Goose" and "slightly introgressed SNGOXROGO", so each records committee member who votes on this species will draw their own line and vote accordingly. (An aside: Michigan no longer reviews this species, which turned regular several years ago.) But in this case, the final answer may differ for different individual birders. I am soliciting the opinions of several experienced observers and will update this blogpost with their opinions once I hear back.

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