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Sunday, January 29, 2006

Cackling Goose question

Today one reader posted this comment:

"Of course everyone is familiar with the average Canada Goose. But the relevant question is: what specific criteria distinguishes the difference between a dwarf Canada Goose and a normal Cackling Goose? Your expert opinion is appreciated."

First, I'm not an expert, far from it. It is a complex issue that I am still learning. Second, the designation of several previously recognized subspecies into a full species have made identification difficult. From what I have read, the bill is a good starting point. Bills are smaller and stubbier than on Canada Goose. Cackling Goose are always smaller than Canada Goose. On average, the largest Cackling Goose is still smaller than the smallest Canada Goose.

There are noticably size differences within the birds of the overwintering flocks in Prospect Park. The individual in question, however, is considerably smaller than any Canada Goose I've ever seen. There is certainly the possibility that our identification of this individual is incorrect and that it is a runt. However, if it were just a runt I would think the bill shape would still look like that of a large bodied Canada Goose. On this point David Sibley writes:

"...Judging from the data here, the difference between the "runt" wild-raised birds from Akimiski Island and their better-nourished counterparts is not enough to cause confusion of these small Canada Geese B. c. interior with Cackling Geese B. h. hutchinsii, for example (see Table 2)..."

I highly recommend reading the complete text of his excellent discussion and identification tips here:

Identification of Canada and Cackling Goose

From the bits that I've learned, so far, it appears that individuals at the extreme ends of the size spectrum are more easily identified. If the bird in Prospect Lake weren't so outstanding in size and bill shape I suspect nobody would have given it a second look. I'm certain that there are cases when even a more experienced birder would have trouble identifying a Cackling Goose among a flock of 500 Canada Goose. I guess we're all going to have to start paying more attention to our most common goose.

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