Variability of Arrival Dates of Maine Migratory Breeding Birds: Implications for Detecting Climate Change

Many authors have recently used changes in arrival dates of migratory breeding birds as a measure of environmental change due to global warming. In this paper, I present a comparative analysis of the intra-annual variability in first arrival dates for 107 species of migratory breeding birds in Maine. Data come from volunteer observers in the southern two-thirds of the state from 1994 through 2005. Using the Julian date for each first arrival, standard deviations were tabulated for each species in each year. The results indicate that some species, specifically leaf-gleaning insectivores and aerial insectivores, have relatively low variance of first arrival dates while other species show more protracted migrations and hence greater variability around the mean arrival date. Post hoc considerations of the patterns of variability suggest that diet may be an important determinant of variance in arrival date. The data indicate that researchers should concentrate on species with lower variances of arrival date to increase the statistical power for testing changes in arrival dates as global warming proceeds.