Print PageWidespread evidence is available demonstrating that shifts in climate are affecting behaviour and phenology in migratory animals. In birds, increasing temperatures near breeding sites are most often related to advances in avian phenology, but conditions at distant tropical nonbreeding areas can also influence the timing of migration and breeding. Typically, higher winter rainfall or temperatures have been correlated with improved body condition, earlier departure from the nonbreeding grounds, and earlier arrival to the breeding grounds. The strength and nature of migratory connectivity might also influence how populations respond to variation in conditions on their nonbreeding grounds. Furthermore, populations could respond to changes in climate through changes in gene frequencies, individual behaviour (i.e., phenotypic plasticity), or both. American redstarts (Setophaga ruticilla) are long-distance migratory warblers that show migratory connectivity at a regional scale. Using long-term datasets, we tested whether arrival and laying dates at two widely separated breeding populations of redstarts were related to rainfall at predicted nonbreeding locations and whether departure dates from a nonbreeding population matched this result. We then assessed whether these results could be explained by changes in individual behaviour.
Results/Conclusions
We found that winter rainfall showed similar associations with bird phenology at separate breeding populations but on a corresponding east-west axis: greater rainfall in Jamaica and eastern Mexico was associated with advanced redstart arrival and laying dates in Ontario and Alberta, respectively. Furthermore, departure dates from a nonbreeding population in Jamaica were associated with arrival and laying dates in Ontario. We found that this variation in redstart phenology was likely a result of phenotypic plasticity, because individual males breeding in Ontario showed changes in arrival that corresponded to yearly changes in rainfall and were similar in magnitude to population-level responses. This is the first study to compare weather conditions from separate regions of the nonbreeding range to migratory bird arrival and laying dates at corresponding breeding populations. In addition, the vast majority of studies to date that have related climate to avian phenology have focused on European species. Our results provide evidence that spatial variation in winter climate could have divergent effects on different breeding populations in species with strong migratory connectivity. Furthermore, such populations might face steep declines as a result of predicted drying conditions in tropical areas due to the tight linkage between early arrival and fitness in long-distance migrants.
