Well there's:

Groth, J.G.1993. Evolutionary Differentiation in Morphology,
Vocalizations, and Allozymes Among Nomadic Sibling Species in
the North American Red Crossbill (_Loxia curirostra) Complex.
University of California Press, Berkeley.

and Jeff Groth's webpage:
http://research.amnh.org/ornithology/crossbills/

ian paulsen wrote:
>
> HI ALL:
> Does anyone have a good reference to the different types of red
> crossbills out there??
>
> Ian Paulsen
> Bainbridge Island, WA, USA
> ipaulsen at krl.org
> A.K.A.: "Birdbooker"
> "Rallidae all the way"

--
Mike Patterson
Astoria, OR
celata at pacifier.com

http://www.pacifier.com/~mpatters/bird/bird.html