Morphometric Variation in Introduced Populations of the Common Myna (Acridotheres tristis): An Application of the Jackknife to Principal Component Analysis

Abstract

Morphometric variation in samples of common mynas from 11 localities in Australia, New Zealand, Fiji and Hawaii was analyzed with principal component analysis. We attempt to avoid two shortcomings in many previous applications of principal component analysis in morphometrics by analyzing separately variation within and among populations, and by applying the jackknife procedure to reduce subjectivity in interpretation of the principal components. Within populations, only principal component I appears to have a stable orientation and this orientation is common to all localities. Component II may be a simple vector, differing across localities, but similarity in the second and third eigenvalues warns that the associated components could be largely arbitrary. The variance along component I does differ across localities. Among populations, again only the first component displays convincing stability, although component II may be a stable but extremely simple vector. Both within and among populations, component I appears to be a general factor representing size and size-related shape variation. The apparently simple patterns of covariation displayed by the introduced populations may be attributable to bottlenecks in small founding populations and the short time since the introductions were made. Future studies should incorporate some form of testing to confirm putative patterns of character covariation, and doing so probably will require sample sizes much larger than has been the custom.