Subject: head width in birds
Date: Dec 9 12:32:43 1994
From: Dennis Paulson - dpaulson at ups.edu
Just a comment about Byron Butler's posting mentioning a wider head in
Ross' Gull as determined by museum study skins. I don't think you can be
sure of this. Parts of the skull may be cut away inside a study skin, and
the amount of cotton stuffed in the orbits affects how wide the head will
look. This comparison should be made only on photographs, live birds in the
hand, or skeletons (even in a skeleton, the eye itself won't be in the
orbit, so you can't determine the exact distance between the eye surfaces,
but you can measure the width of the interorbital area to get an idea of
how wide the head is).
In any case, I would doubt that Ross' Gull "needs" better binocular vision
than any other gull, as they all feed so similarly and all clearly have a
need to be able to judge distance (depth perception is the commonly
attributed gain from binocularity). Looking at the Audubon Master Guide
photos again, I can't see that Ross' Gull looks particularly broader-headed
than the other small species. But, as I wrote before, its eyes look a bit
larger, so the distance from eye surface to eye surface may indeed be
greater than in a Bonaparte's.
I hope this is also an object lesson to all who don't understand why the
collection of birds is important; there still aren't enough bird specimens
preserved, even in big collections such as the Peabody Museum! There is no
Ross' Gull skeleton anywhere in the Northwest, as far as I know, and only a
couple of skins. There were only 20 Ross' Gull skeletons in museums as of
1986, and only a few museums actually have series of skins of this species.
Dennis Paulson phone: (206) 756-3798
Slater Museum of Natural History fax: (206) 756-3352
University of Puget Sound e-mail: dpaulson at ups.edu
Tacoma, WA 98416