INTRASPECIFIC VARIATION IN THE HINDLIMB MUSCLES OF THE IVORY-BILLED WOODCREEPER AND THE BLUE JAY, WITH A REVIEW OF OTHER SPECIES

Systematists have long used myology in phylogenetic studies of birds, but have only recently begun to assess the extent of intraspecific var iation in muscles and the potentially disruptive effect of such variat ion on investigations of phylogeny. We provide information on the Ivor y-billed Woodcreeper (Xiphorhynchus flavigaster) and the Blue Jay (Cya rocitta cristata), and integrate these results with those of previous studies. Bilateral dissection of 19 specimens of X. flavigaster reveal ed seven major variants in six different muscles. Ten of the 19 specim ens (52.6%) possessed variant muscles; of these, four were unilateral and seven bilateral. Overall, 1.6% of the individual muscles varied fr om their species-typical states. Six of the 11 variants were found in M. flexor cruris lateralis, suggesting an association with that muscle . The 26 specimens of C. cristata exhibited a different pattern. Major variations occurred in only three muscles, but for each of these a ty pical condition could not be ascribed because variation was extensive and polymorphic. Analysis of similar information for a total of 14 spe cies of passerine birds indicated that variation is common enough that many individuals in a sample can be expected to possess at least one variant muscle. However, the probabilities of error in designating spe cies-typical conditions from bilateral dissection of single specimens were low (0.00-0.06), suggesting that intraspecific variation does not significantly compromise phylogenetic analyses of myological data. Si mulation experiments are needed to test this hypothesis.