I thought this would be of interest to the tweeter community, too.

>Dr. Paulson,
>
>I saw your comment on Tweeters that the redpoll is 2/3 the size of the
>house finch. Since you no doubt have a drawer full of these guys, you
>are the authority. Since I've never seen a redpoll, I looked it up
>in my guide books, and found that both the Peterson and Stokes field
>guides indicate that the redpoll and the house finch are the same size.
>Perhaps they should be alerted that their data are wrong?
>
>Bob Mauritsen

Bob, you're right in assuming I can go look at the relative size of
redpolls and house finches any time I wish, from large series of them in
the collection. I don't have a clue why bird guides so often get sizes of
birds wrong, but they sure do. I rarely look at the length measurements in
field guides, so I don't even notice the discrepancies until someone calls
one of them to my attention (or uses it inappropriately to distinguish two
species). I guess someone could keep busy for quite a few months calling
attention to these incorrect measurements! I had a master birder class
look through field guides once to compare how differently the birds were
described in the different books, and it was a bit shocking to see how much
variation there was just in the length figures.

The main problem my size assessment and that of the books is *so* different
is because I always think of bird weight rather than length as a measure of
body size, and field guides always use length as the measurement. Average
weight of a House Finch is 21 grams, Common Redpoll 13 grams, thus redpoll
is actually 62% of the size (=weight) of the finch (my "two-thirds" was
just a guess). For years I've been pushing the idea of having size
indicated by weight in guides, as I think length measurements have the
potential to confuse people because a long-tailed bird ends up with a
larger number in the "size" category in the book than a bird that is quite
obviously larger but with a shorter tail. And that's quite independent of
whether they get the figures right or not!

Dennis Paulson, Director 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
web site: http://www.ups.edu/biology/museum/museum.html