Parallel Evolution and Bird Niches

Abstract

The chief aim of this chapter is to examine the extent to which bird niches show similarities in response to parallel selective forces. The problem can be approached at various levels, for parallel evolution can be demonstrated within a broad range of sample sizes or units on each of two variables. The first is a gradient of decreasing proximity of habitat sites, so that initially comparisons may be made within habitats, then between adjacent habitats, and extended to comparisons on a continental and finally an intercontinental scale. A second axis ranks various sample sizes of numbers of bird species used in these comparisons. The comparisons may be made at the level of the single species, of groups of related or coexisting species, of whole communities or of the complete avifauna. The particular emphasis of this review is the comparison of communities, a grouping of species at an intermediate to higher level of organization, between continents, the broadest possible scaling of habitat isolation. At these levels parallel evolution is at its most dramatic, for the former choice extends the matching of individual species niches to include the relative juxtaposition of these niches in the community, their number and shape, and the latter condition assures us that the species in these communities so compared will bear minimal taxonomic affiliation to each other and thus minimal resemblance due to factors other than parallel selection. While comparisons at different levels of species organization and geographic separation will be treated in turn as available information allows, the intercontinental community comparisons will be given special attention. I have studied several sorts of avian communities, especially in North and South America; this information will be presented and the community organization contrasted. In particular, the Californian and Chilean bird communities of broad-leaf sclerophyll scrub will be discussed.