[obol] Re: Taverner's Cackling Goose vs. Lesser Canada Goose
- From: Wayne Hoffman <whoffman@xxxxxxxx>
- To: David Irons <llsdirons@xxxxxxx>
- Date: Wed, 15 Oct 2014 10:09:41 -0700
Hi -
A few thoughts about all of this. First, this discussion is happening
because it is important to tell the different forms of white-cheeked geese
apart, and because it is not particularly easy. Different forms have very
different conservation status. Hunters are expected to be able to
distinguish them, and we as birders ought to be able to do at least as well
as they do, both as a matter of ego, and so we can assess the
successes/failures of management.
Second, two important questions underlie the whole discussion. Most of the
conversation and linked resources so far have been directed at the
question, "What are the distinguishing features of the different forms?"
This is appropriate, but the other question is also very relevant, "How
did these birds become differentiated into so many forms?" I'll try to
provide the beginnings of an answer.
So, Three: Northern Geese have a series of life-history traits that
together promote differentiation of local or regional forms. These include
the following: a) they migrate in family groups of parents plus juveniles,
assembled into flocks of birds that breed in the same local areas; b) these
flocks tend to stay together through the winter and through the spring
migration; c) they are mostly monogamous, and choose mates while wintering,
mostly within these flocks of birds from the local areas; and d) they have
the capability of adapting their morphology quite rapidly to the specifics
of their breeding habitat and migration length.
Four: Most of the morphological characters that we use to tell them apart
are these same things that adapt to habitat and migration. Generally, size
is negatively correlated with migration length: The geese that nest
farthest north tend to be smallest, and the biggest ones have the shortest
migrations. Among the white- cheeked forms, the tundra-nesting ones
(Cackling Geese) are small, the biggest are the "Giant Canada Geese" which
have very short migrations if any, and among the other Canada Goose forms,
ones with longer migrations (e.g., Dusky Canada Geese) are smaller than the
more resident ones (Western Canada Geese). This also holds among species:
Brant nest at high latitudes, have long migrations, and are very small.
Ditto Ross's Geese. Snow Geese and White-fronted geese have relatively
long migrations for their size, but tend to break them into segments with
longer stopovers. Bill size and shape, neck length, and plumage color also
are subject to fairly rapid adaptation.
Five: Now getting back to identification. "Real" Cackling Geese (ssp.
minima) have a compact breeding range in coastal Alaska. Aleutian Cackling
Geese have a larger range currently, but went through a bottleneck a few
decades ago, where the breeding range was tiny (Buldir Island plus the
Semidi Islands). In contrast, "Tavernier's" Cackling Geese have a huge
and ecologically diverse range, and are substantially more variable. So a
big part of the ID problem is that Tavs do not all look alike, They are
more variable in size, color, and bill shape than either Aleutians or
minima Cackling. Ditto for Lesser Canadas. I think we have a tendency to
identify Tavs by process of elimination: If it is too big, too pale, too
long bill to be minima, then it must be a Tav...
Wayne
On Wed, Oct 15, 2014 at 12:03 AM, David Irons <llsdirons@xxxxxxx> wrote:
> 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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