Biogeographic patterns of passerines during the breeding season.

Abstract

The results of numerical analyses examining biogeographic patterns are often the delineation of distinct regions or provinces based upon the cumulative distributional limits of each member of a respective taxon. Resulting patterns among groups of species, or assemblages, can then be used to shed light on how species assemblages correspond to broad-scale environmental and habitat features. These same biogeographic patterns also provide opportunity to examine how historical factors such as climate change and vicariant events have shaped species distributions. By virtue of their mobility and conspicuous behaviors, birds represent an easily observed and recognizable group. As a result, a great deal of information has been collected regarding the geographic occurrence of most North American bird species. The objective of my study was to examine the major biogeographic patterns of passerines during the breeding season across the continental United States. Data regarding the breeding distribution of nesting passerines were collected from checklists obtained from the extensive National system of parks, refuges, and reserves scattered across the contiguous United States. Presence/absence data compiled from checklists were converted to a standardized data matrix based on the Coefficient of Jaccard. The data matrix was then submitted to cluster analysis and principal components analysis. Results of these numerical analyses revealed five major avifaunal regions based upon passerine distribution in the continental United States. These regions are largely coincident with certain broad physiographic regions; 1) northern hardwood/coniferous forests and associated grasslands, 2) eastern and southeastern forests and associated grasslands, 3) the Great Plains, 4) mountain ranges and intermontane regions of the western United States, and 5) the southwest. The fourth and fifth areas are relatively species-rich and, along with central and southern sections of the Great Plains, form one of the three major branches of the block dendrogram produced by cluster analysis. The passerine avifaunas of the central and southern plains were most similar to those of the American southwest and west. The northern plains, in contrast, grouped more closely with the passerine avifaunas of the northern/northeastern United States, comprising a second branch in the dendrogram. The third branch consisted of the southeastern United States and southern Texas. Differences among these regional passerine avifaunas were largely based upon similarities in distribution among several species adapted to gradients in regional environmental and ecological conditions.