Temporally increasing spatial synchrony of North American temperature and bird populations

The ecological impacts of modern global climate change are detectable in a wide variety of phenomena, ranging from shifts in species ranges to changes in community composition and human disease dynamics1–3. So far, however, little attention has been given to temporal changes in spatial synchrony—the coincident change in abundance or value across the landscape4—despite the importance of environmental synchrony as a driver of population trends and the central role of environmental variability in population rescue and extinction1,5,6. Here we demonstrate that across North America, spatial synchrony of a significant proportion of 49 widespread North American wintering bird species has increased over the past 50 years—the period encompassing particularly intense anthropogenic e ects in climate—paralleling significant increases in spatial synchrony of mean maximum air temperature. These results suggest the potential for increased spatial synchrony in environmental factors to be a ecting a wide range of ecological phenomena. These e ects are likely to vary, but for NorthAmericanwildlife species, increased spatial synchrony driven by environmental factorsmaybe thebasis for apreviously unrecognized threat to their long-term persistence in the form of more synchronized population dynamics reducing the potential for demographic rescue among interacting subpopulations. The quest to understand mechanisms driving the dynamics of natural populations has been a key motivation for plant and animal population studies for over a century. Among the more intriguing patterns exhibited by a wide range of taxa is spatial synchrony—the coincident change in abundance (or other time-varying character) of geographically disjunct populations4. Spatial synchrony is widespread among ecological phenomena and can be driven by a variety of factors, including dispersal7, mobile predators8,9, and environmental drivers (the Moran effect; refs 10,11). It is also thought to play an important role in metapopulation dynamics, primarily because increased synchrony among interacting populations reduces the probability that poorly performing populations can be demographically rescued by emigrants from successful populations, thus increasing the threat of extinction5,6,12,13. Consequently, it is of considerable importance to measure the degree to which spatial synchrony is changing as well as whether such changes can be attributed to specific environmental patterns14,15. Here we address this question using 50 years of wintering bird and environmental data from a large number of sites across North America measured between 1960 and 2009 (Fig. 1). We first examined a subset of 49 widespread wintering North American Figure 1 | Locations of North American bird survey sites and weather stations used in the analyses of spatial synchrony. Black crosses are sites of Christmas Bird Count circles (N≈550 sites) and grey dots are sites providing 40 or more years of temperature (N≈ 1,350) and/or rainfall (N≈ 1,005) data between 1960 and 2009.