The Influence of Climate and Population Size on the Distribution of Breeding Dates in the Red-Backed Shrike (Lanius collurio)

There are now a growing number of studies linking environmental conditions operating at different life stages of birds to their arrival on breeding grounds. Here we focus on one of the major fitness determinants; timing of breeding. We examined the influence of climate relevant to different parts of the birds' annual life cycle, and the impact of population size on distribution traits in a central European population of the red-backed shrike Lanius collurio. Timing of breeding was affected by breeding ground climate as well as by population size. In years with higher population densities shrikes started to breed earlier. In contrast to previous studies we did not find that our climatic variables adequately explained the variability of timing of breeding. We argue that density-dependent processes are also important for the reproduction of shrikes and that in phenological studies; attention should also be paid to other factors operating at breeding grounds.