Thursday, November 03, 2005

Probable Ross's Goose X Lesser Snow Goose

(click on image for larger version)

It's November and you find a small white goose on a pond in your neighborhood. It's even smaller than a Cackling Goose that paddles by. It has a small bill, but is it small enough? There is a slight grin patch and the Sibley Guide seems to suggest variability with this field mark. Does that mean other field marks can show variability within that species? The question of identification seems clear: Snow Goose versus Ross's Goose.

This mystery goose seen at Stricker's Pond in Middleton this past weekend fooled several experienced birders, myself included. It was initially reported to the Wisconsin Bird Network as a Ross's Goose, and since it was close by, Jesse Peterson and I decided to look. The first hint of a puzzle came at the scene when another seasoned birder stated, "I just can't turn this into a Ross's Goose." Given the small size of it, Jesse and I thought it was unquestionably a Ross's, but I still digiscoped it for closer study at home.

The group of birders who first reported this bird as a Ross's Goose had also taken photographs and posted them to the Wisconsin Birding Network. The identification was further scrutinized and Milwaukee's John Idzikowski came to the rescue:

"The white goose in this shot shows a great deal of intermediacy with Snow Goose showing a small grinning patch and a bill shape pattern where the cheek feathering rounds in towards the tip of the bill; this should be straight in Ross's with no or only a very small grinning patch. The bird overall is not very petite with a clean small triangular bill as expected for Ross's and while we have only a poor context of size next to what is probably a mid-size or giant Canada Goose, it seems too large and the neck too long and thick.

I remember when Ross's were truly rare occurrences in migration in Wisconsin and very few birds were recorded each spring. In the last 30 years we have seen an increase in Ross's ID's; how many of these are smaller but still hybrid Ross's x Lesser Snow and not true Ross's? Ross's has expanded its range to the east in the last 50 years and now nests along Hudson Bay having moved into the range of Lesser Snow due north of Wisconsin, probably with an increase in hybridization - so there's more value to this exercise than just challenging the ID."
Link: Discussion of Ross's X Snow from Cornell

It may not have ended up on my year list, but it was still a neat bird and also a great lesson!

Hybrid goose image © 2005 Michael McDowell

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