Patty and others,

I recently wrote the following review that is still in press as far as I
know:

Thompson, C.W. 1998. Molt and nuptial color. Pages 285-295 In E. Knobil
and J.D. Neill (eds.) Encyclopedia of reproduction. Vol. 3. Academic
Press, New York.

It is the most comprehensive and up-to-date review of the role of
hormones in the expression of soft part and plumage coloration. I'd be
happy to send a preprint to anyone who would like a copy. Patty, if you
are specifically interested only in changes in bill color, I can supply some
references for that.

Best,

Chris Thompson, Ph.D.
Research Scientist
Washington Dept. of Fish and Wildlife