Species Relationships and Adaptations for Sympatry in the Avian Genus Vireo

Abstract

This paper reviews species relationships within the New World avian genus Vireo and draws some inferences on the origin of sympatry among members of the group. It is presumed that the present-day distribution, habitat ecology, and external morphology of these species might offer some information relating to the evolutionary history of the genus. Although species of vireos are well known to American bird systematists, little attention has been directed to the genus as such. It is hoped that the present report will serve as an outline to the numerous biological problems that are unresolved for this group, and that it will encourage others to use the approach of comparative systematics in the study of subgeneric affinities. Specifically, this paper examines the following topics about the genus Vireo: (a) the distribution of species-specific characters of the external morphology, habitat preference, and foraging ecology throughout the several subdivisions of the genus, and the bearing of these data on the delimitation of subgenera and species groups; (b) the factors which may be responsible for the degree to which isolates or species have diverged from parental stocks while in allopatry; and (c) the manifested adaptations, presumably acquired in isolation, whereby the species can avoid competition and thus attain high degrees of sympatry. I have also used the apparent relationships in the genus Vireo as a point of departure for theorizing about the formation of species-specific characters and the development of sympatry. In consideration of the latter topic, it was found particularly useful to compare affinities in Vireo with those in the titmouse genus Parus; the papers of Hinde (1952), Gibb (1954), Snow (1954), and Dixon (1961) have been consulted especially concerning adaptation in the latter genus. I have already reported on trends of intraspecific variation for the widely distributed species of Vireo (Hamilton, 1958). That information will not be duplicated here except as it relates to the general problem of adaptation and divergence of members of the genus.