Resolution of Cryptic Species in Appalachian Red Crossbills

Two distinct and sympatric forms of Red Crossbill (Loxia curvirostra) from breeding populations in Virginia and North Carolina were resolved using multivariate analysis of vocalizations and morphology. Five characters were used to describe individually distinctive flight calls of 135 adults which were also measured for a set of nine bill and body size characters. Call notes were bimodal for two characters, and a principal components plot separated birds into two clusters based on call note shape. Univariate distributions of morphological data showed some weak bimodalities and higher coefficients of variation than other passerine bird populations. Distributions along multivariate first principal component axes from morphological data (sexes in separate analyses) were bimodal, dividing fully grown adults into two distinct bill and body size classes. The vocal and morphological multivariate clusterings of individuals were entirely congruent, but no univariate character in either data set could completely separate the two forms. The matrix correlations between vocal and morphological interindividual distances were 0.441 for males and 0.423 for females and were highly significant using Mantel tests. However, within each hypothetical cryptic species, vocal and morphological characteristics were uncorrelated, showing a lack of pattern in the sample beyond the division into two nonreducible clusters. Other vocal differences between the two forms are described, and observations of ecological differences are summarized. The type specimens representing the two names which have been applied to Appalachian crossbills, L. c. pusilla (Gloger 1834) and L. c. minor (Brehm 1845), were compared to birds of known vocalizations. The type of pusilla matched the larger Appalachian form, but the type of minor was smaller than all adult males in the sample. The taxonomic problem of L. curvirostra is discussed, and an argument is made in favor of the species level for the two Appalachian vocal and morphological forms.