Tuesday, November 4, 2008

I vote for parvipes

I've been putting a lot of thought (yeah, yeah, entirely too much) into these ridiculous small geese.  I had hoped today to find a snowy owl that's been reported close to home, though is not pinned down to any one location over about a 8-10 square mile area it's been reported from.  I didn't find the owl.  What I did find was more small geese!  Clearly the fates were with me in that they're from the group that has been causing me problems: small plain birds the same color as Canadas without bright scapular edging.

I spent a lot of time in the last blog discussing whether small runty interior Canada geese (the common migratory form in Michigan) could be part of the cause for confusion.  The small birds, however, are all about the same size.  Unless dwarf geese are common in that subspecies for some genetic reason, if the small birds are small because of poor habitat where they're fledged, then there should be a range of sizes which there isn't.  Though I maintain that the support that David Sibley gives based on his interpretation of the data for runty interiors being unlikely to cause confusion with hutchinsii  Cacklers isn't entirely valid, it seems unlikely that he would write something that doesn't jive with his experience (which obviously is quite valid).  Therefore, if these birds aren't all small interior Canadas, and they aren't hutchinsii Cacklers that means they're something else.

The Michigan Bird Records Committee 2005 report (viewable here) discusses the specimen record in Michigan.  The University of Michigan Museum of Zoology has specimens of several hutchinsii Cacklers ... and one "assigned to the subspecies taveneri."  Tavener's Cackling Goose is a bird of the Arctic coast of Alaska and winters in the Northwestern U.S.  It is also the subspecies of cackling goose closest to the parvipes Lesser Canada goose, and some authorities apparently consider them difficult, if not impossible to separate.  I think this would lead me to suspect that a 4th white-cheeked goose occurs in Michigan, the parvipes Lesser Canada Goose.

I did some google searches and came up with a couple of enlightening webpages from birders in the western U.S. in the expected wintering range of parvipes Lesser Canadas.  This excellent one from Denny Granstrand in Washington has nice photos of most of the Cackling goose subspecies as well as the western Canada goose subspecies with notes on ID.  Harry Krueger's from Idaho further details ID of parvipes Canadas and hutchinsii Cacklers.  The biggest general difference they describe is that hutchinsii Cacklers have very square heads with very vertical foreheads (think more like a Barrow's Goldeneye) whereas parvipes Canadas have rounder heads whose foreheads slope more evenly into the bill (think more like a canvasback).  If you pull out your big Sibley, you'll find this illustrated as well (although again not field marked).

Here's my two geese from today.  You see a forehead that slopes into the bill (admittedly somewhat accentuated by the positioning):

The next 2 photographs show 2 things, first the effect that the birds in the background of a digi-scoped (or any shot taken with a big lens) will appear relatively larger.  The small goose in the first photo is not dwarfed by the nearby presumed interior like it is in the second.  To me the patterning in the scapulars seems to change somewhat with the lighting, appearing slightly more prominent in the sidelit sun but slightly less prominent in the shade:

Finally another view of the scapulars in direct sun showing little edging to them (note that the interior it's with has a weak white crescent at the base of the neck, something they don't often show - though don't make that mark a priority of study in the field if you have limited time to view an individual, almost everyone agrees it's a variable feature in all populations)
The bottom line, is that I think that parvipes lesser Canada occurs in Michigan, and accounts for a lot of the small brown white-cheeked geese seen in late October and early November.  It seems that the shape of the forehead provides a lot of the sense of a bill's relative "stubbiness," so add vertical forehead for cacklers to the vertical bill base for Ross's goose for things to study in small geese.

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