Groth, J. G.  1993.  Evolutionary differentiation in morphology,
    vocalizations, and allozymes among nomadic sibling species in
    the North American red crossbill (Loxia curvirostra) complex.
    Univ. California Publ. Zoology, vol. 127.

It contains hundreds of audiospectrograms, measurements of beaks and bones, results of a detailed genetic survey, and extensive natural historical, and behavioral data. It can be found in university libraries, or may be obtained from the publisher (I think it's $17):

    The University of California Press
    2120 Berkeley Way
    Berkeley, California  94720
    phone  (510) 642-4247
    FAX    (510) 643-7127
    email  [email protected]