Emmanuel Paradis [1,2,3]
Stephen R. Baillie 
William J. Sutherland 
Richard D. Gregory [1,4]
Abstract. 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 ofthem 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 assessedits 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 onespecies 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 farmlandscompared 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 synchronyis 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 populationsynchrony is more intense during declines.
Key words: birds, population synchrony, Britain; Common Birds Census; cross-correlation, farmlands and woodlands; habitats; population index; population trend; spatial scale; synchrony.
The spatial distribution of living beings is widely recognized as being crucial in population dynamics (Tilman and Kareiva 1997). A critical issue in thisperspective is the pattern of synchrony in local population fluctuations. Species with synchronous populations are thought to face greater risk of extinction because density crashes can occur simultaneously in all populations (Heino et al. 1997). Synchrony among populations is also thought to be critical for population stability (Ruxton 1994, Paradis 1997).
Population synchrony can be characterizedby its intensity (measured typically by the correlation between two population time-series), and its spatial extent (the relation between intensity of synchrony and distance between populations). Recent studies have attempted to characterize the spatial extent of population synchrony for several species. Evidence has been found for widespread synchrony in moths and aphids (Hanski and Woiwod1993), snowshoe hares (Lepus americanus) (Sinclair et al. 1993), and tetraonids (Lindstrom et al. 1996). In each case the authors concluded that weather is a likely candidate as a synchronizing factor. Contrasting results in which significant synchrony occurs only at a limited spatial distance, populations far apart fluctuating independently, were shown for a rodent, Clethrionomys glareolus (Steen et...