Thursday, March 30, 2017

Ross’s Goose

On Tuesday, Erika and I discovered seven Ross’s Geese at the Dennison Water Treatment ponds in Rice County. Its not every woman who gets dates to sewage treatment ponds! Ross’s Geese are very similar to Snow Geese. The Ross’s Goose’s bill, however, is much shorter than a a Snow’s, the bill lacks a black grinning patch (the line running along the top of the lower mandible), and Ross’s usually show blue warts at the base of their upper mandibles.

In the final photo, you can see the longer, black-ribbed, and non-warty Snow Goose bill. The bird to the right is problematic, since it is clearly larger than the Ross’s Goose to the left.  It does not help that these birds grazed between me and the sun, but, enlarging the photo, seems to show the blue wart field mark. Male Ross’s Geese are, in fact, six percent larger than the females, which may explain the size difference.

The two species also hybridize. In the photo of the Snow Geese (taken years ago in South Dakota), you see both color phases of Snow Geese—white and blue. The blue form is apparently the result of recessive genes in Snow Geese. Very rarely, blue Ross’s Geese are reported. Recent research indicates that  Ross’s Geese lack the blue allele. The blue forms, therefore, are thought to be hybrids between Snow and Ross’s geese breeding back with Snow Geese (Jonsson et al. 2013).

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