Winter Biogeography of North American Fringillidae (Aves): A Numerical Analysis

Bock, C. E., J. B. Mitton, and L. W. Lepthien (Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder 80309). Winter biogeography of North American Fringillidae (Aves): a numerical analysis. Syst. Zool. 27:411-420.-Winter distribution and abundance patterns were generated for 65 species of fringillid songbirds, using results of 2,732 Christmas bird counts grouped into 59 latitude-longitude blocks. Similarity coefficients were computed among the blocks, based upon the species occurring in each block and their abundances. Similarity matrices were subjected to cluster and principal components analyses. Results showed four major biogeographic regions: the north, mid-latitudes (with some east-west separations), the southeast, and the southwest. With analyses of abundance, areas of high similarity were those in which the same fringillids were the commonest species in the blocks; in more traditional analyses of presence-absence data, similar areas were those which shared large numbers of species regardless of their relative abundances. Because of these differences, abundance data showed the northern and southeastern regions as larger and more distinctive, while presence-absence data emphasized the extent and patchiness of the southwestern region. Eastern and western latitude-longitude blocks were regularly and increasingly dissimilar with decreasing latitude, a result best shown by principal components analysis of abundance data. This east-west division occurred along the 100th meridian, and the southernmost block along this boundary (southern Texas) had a highly distinctive winter fringillid avifauna. [Biogeography; Fringillidae; cluster analysis; principal components; winter; abundance patterns.] This study describes avian biogeographic regions of North America, as revealed by winter distribution and abundance patterns of birds in the family Fringillidae. We selected this group for two reasons. First, it is a diverse family, well represented in virtually every terrestrial habitat (e.g., Shugart and Patten, 1972). Second, there is ecological and morphological evidence suggesting that the winter season is critical for these birds (Fretwell, 1968, 1969), so that their winter habitat preferences would be finely tuned by strong selection pressures. While breeding ranges of birds have been used most frequently in studies of avian biogeography (e.g., Udvardy, 1963), there is no reason to ignore winter patterns a priori. Indeed, it is only in a highly mobile group such as birds that we can hope to factor out the influences of winter versus summer seasons upon the biogeographies of species or groups of species. North American patterns of avian species diversity are more striking in winter than they are in summer (Bock and Lepthien, 1975). This appears to be a reflection of steep environmental gradients that characteristically are found during the winter season (Bock and Lepthien, 1974; Tramer, 1974a, 1974b). These gradients should have an equally strong influence on the location and distinctness of the biogeographic regions we have attempted to describe. We have analyzed data extracted from the results of Audubon Society Christmas bird counts (see Methods). Count results were grouped into 59 blocks of latitude and longitude (Fig. 1). These blocks then served as operational taxonomic units (OTUs; Sneath and Sokal, 1973) for cluster and principal components analysis. Some biogeographers have used cluster analysis to define regions or provinces in North America (Hagmeier and Stults, 1964; Hagmeier, 1966; Kaiser et al., 1972), while other workers have preferred principal components analysis or factor analysis (e.g., Fisher, 1968; Thaler and Plowright, 1973; Stevenson et al., 1974; Schnell et al., 1977). Regardless of