[obol] Taverner's Cackling Goose vs. Lesser Canada Goose
- From: David Irons <llsdirons@xxxxxxx>
- To: OBOL Oregon Birders Online <obol@xxxxxxxxxxxxx>
- Date: Wed, 15 Oct 2014 07:03:22 +0000
Greetings All,
Over the past seven years or so, I have spent a considerable amount of time
studying and photographing the smaller forms of white-cheeked (Canada/Cackling)
geese that winter in the Pacific Flyway. While most of these efforts have been
made within what might be referred to as the Willamette Trough–from Ridgefield
NWR south to Eugene–I have also studied Aleutian Cackling Geese in Humboldt
County, California and spent a fair amount of time looking at the birds that
winter in pastures along the lower reaches of the Nestucca River in southern
Tillamoook County.
Earlier in the day Bob Archer, Scott Carpenter and others have offered some
good tips for separating the two predominant forms of Cackling Goose in the
Pacific Flyway. Ridgway's Cackling Goose (subspecies minima) now winters mostly
in the Willamette Valley and extreme southwest Washington. Aleutian Cackling
Goose (subspecies leucopareia) is for the most part a coastal migrant in
Oregon and northwestern California and then turns inland to winter in
California's Central Valley.
You will be hard-pressed to find any source that offers meaningful criteria for
separating the third Pacific Flyway form of Cackling Goose (Branta hutchinsii
taverneri) from Lesser Canada Goose (Branta canadensis parvipes). In fact, most
descriptions of these two taxa compare them to one another and describe them as
being the same size and virtually identical in appearance. In the ID pdf shared
earlier today by Bob Archer, the culmen measurements offered for both forms
(32-40mm) are identical. I've photographed birds that I initially presumed to
be Lessers and other birds that I initially presumed to be Taverner's Cacklers
and compared lots of birds in the field. I've never been able settle on any
differences other than those that fall within a cline that I think can be
attributed to individual variation. Based on what I've seen, the range of
variation in the "Tav/Lesser" grouping mentioned by Scott Carpenter is
certainly no more extensive than the variation that I see among minima
Cacklers. I find it particularly telling that an otherwise accurate and
authoritative treatment of the ID criteria for white-cheeked geese (the pdf
shared by Bob) includes not a single comparative of image of Lesser v.
Taverner's, even though both forms supposedly winter side-by-side in the
Willamette Trough.
In my opinion, the voice of the putative subspecies Taverner's is honking and
more Canada Goose-like and not at all similar to the higher-pitched yelping
vocalizations of other subspecies of Cackling Goose. This, along size
differences between Taverner's and the three other Cackling Goose subspecies,
suggest to me that what we know as Taverner's Cackling Goose may be more
closely related to the Canada Goose group. Taverner's are significantly larger
than the the three other forms of Cacklers and they are close in size to Lesser
Canada Goose and Dusky Canada Goose. Given that Taverner's and Lesser breed and
winter in the same areas, look alike and sound alike, what criteria should we
use to sort them? I recall reading somewhere that a mtDNA study showed them to
be distinct subspecies, but I haven't the foggiest notion of how one might
separate them in the field.
As Scott mentioned, even after much study there are birds that can't be safely
assigned to subspecies. I routinely see "tweeners" that I can't confidently
slot to subspecies. That said, I think that the majority of Ridgway's,
Aleutian, and Richardson's Cackling Geese can be identified in the field.
However, separating Lessers and Taverner's in the field remains a challenge
even for the most experienced goose observers. I certainly can't claim to be
able to ID them visually.
Dave Irons
Portland, OR
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