In Ian Newton's classic book, Population Ecology of Raptors, he has a
chapter on the female/male size difference. He looks at about 5 different
theories on the difference, discards all of them and does not offer one of
his own. It has been several years since I have read this book so maybe
somebody with a more recent reading has a better sense of it. I would hope
that if any other tweeters has some light to shed on this question they
will share it.

When I was a student at San Jose State University some 20 years ago the
professor who owned a copy of this book would not lend it out. If you
wanted to read it you had to come to his office and sit in his office while
he was there and read it. It had already been stolen from the school
library. I eventually got my own copy. Great book.

It is also interesting that the female/size difference exists in owls and
skuas. Also I read that the difference in some species varies with the
diet. In Kestrels, if I remember correctly, the difference in size
increases as the prey items increase in size. So there are regional
variations in the size difference.

Jim

>
>I am not on tweeters currently, but have a question about the different in
>size of adult male and female raptors. Is it known why females are
>larger? Please respond to me personally. Thanks for any light anyone can
>shed on this.
>
>Carolyn Eagan
>eaganc at seanet.com
>
Jim Rosso
Issaquah, Washington
425-392-8440
jrosso at mediaseek.com