Geostatistics as a Tool for Examining Hypothesized Declines in Migratory Songbirds

Data from the North American Breeding Bird Survey (BBS) indicate significant declines in the populations of several species of songbirds, including several Neotropical migrants. These declines have been attributed to habitat destruction and fragmentation on the breeding grounds, in strategic migratory stopover sites, and on the wintering grounds. Using BBS data from the 1967—1989 period and universal kriging, we produced maps of abundance change for two declining species of wood warblers to test hypothetical spatial scenarios of decline over entire breeding ranges. These species were the Cerulean Warbler (Dendroica cerulea) and the Prairie Warbler (D. discolor). We found considerable variability in the location of areas of decline when comparing successive 5—yr periods. In some comparisons, areas of decline were concentrated in the centers of abundance of these species, and in others, they were scattered throughout their range. We also found that the direction and intensity of population trends was quite sensitive to the methods used for calculating abundance. Our results indicate that, even for species where significant long—term declines have been reported, considerable variation exists in the direction of abundance change, both geographically and temporally. Although most of the long—term declines reported in particular species at local and regional scales are undeniable, gaps in our knowledge still prevent us from incorporating these trends into a global model of the annual cycle of Neotropical migrants.