Variation in the size and shape of Darwin's finches

Abstract

Six bill dimensions, and wing, tarsus and hallux lengths were measured on almost all museum specimens of Geospiza species available, and up to 20 specimens from each population of the remaining species of Darwin's finches. The data were subjected to univariate and multivariate analyses in order to provide a quantitative description of size and shape differences among populations and between species. Each species of Geospiza varies among islands in size, and most of the remaining species do so as well. There is more variation in shape among species than among populations of the same species, especially in bill proportions. Allometric relations differ among species. Approximate morphological counterparts to the ground finch species, Geospiza, can be identified among the tree finches. There is a small amount of overlap in multivariate space between a ground finch species and a tree finch species (two cases), but no overlap between any two species within each group. Size variation among populations is not generally correlated with geographical variables such as latitude, longitude, island area or its degree of isolation. Nor do coefficients of variation show strong geographical trends. Several of the results confirm the findings of other workers from simpler and non-statistical comparisons. In addition we have shown that the tree finches have relatively long legs (tarsi), and that these finches as well as the ground finches which spend most time scratching on the ground or climbing in cacti also have a relatively long hallux (hind toe). To interpret the various morphological patterns a knowledge is required of inter-island variation in food supply, feeding habits and the incidence of genetic exchange between populations. Recent field studies have provided some of this needed information, which helps to explain, among other things, why several populations of Darwin's finches are so unusually variable.