Résumé
The degree of synchrony between populations is critical for their dynamics at large spatial scales. Population synchrony has been assessed for only a few species of vertebrates, most of them have largely fluctuating populations with multiannual cycles. We investigated the intensity and spatial extent of synchrony among populations of birds in Britain. The data analyzed ran from 1962 to 1995 at more than 1000 local sites and concerned 60 species. For each species, we measured the intensity of synchrony among local populations using the cross-correlation function (CCF) and assessed its spatial extent. We tested for differences in these two measures with respect to habitat. We also assessed the potential influence of long-term trends in population synchrony. By aggregating the sites in regions and estimating an index for each region, we measured synchrony at a large spatial scale. In general, we found that synchrony was low between populations of British birds. All but one species with the strongest level of synchrony show a pronounced long-term decline. The mean CCF was significantly correlated with the mean abundance of each species in the Common Birds Census (CBC) sites, suggesting that the more abundant species have more synchronous populations. Significant differences in synchrony between habitats were found in 25 species; synchrony was stronger in farmlands compared to woodlands for 22 species. The same result was observed both at the intra- and interspecific levels. This may be the result of national change in agricultural practice affecting all farmland populations. Our cross-scale analysis showed that synchrony is a scale-dependent phenomenon. Our study is the first to focus on a large number of "noncyclic" species, and it suggests that synchrony is weaker for these species than for those with widely fluctuating cyclic populations. This is in agreement with some studies on synchrony of insects. Synchrony in natural populations seems to be determined by complex interactions between abundance, population variability, species characteristics, and demographic mechanisms. A general pattern emerging from our study and others is that population synchrony is more intense during declines.