Nonlinear Impact of Climate on Survival in a Migratory White Stork Population

1. There is growing evidence that ongoing climate change affects populations and species. Physiological limitation and phenotypic plasticity suggest nonlinear response of vital rates to climatic parameters, the intensity of environmental impact might be more pronounced while the frequency of extreme events increases. However, a poor understanding of these patterns presently hampers our predictive capabilities. 2. A recent climatic shift in the Sahel, from droughty to less severe condition, offers a good opportunity to test for an influence of the climatic regime on the response of organisms to their environment. Using a long-term capture-mark-recapture data set on a white stork (Ciconia ciconia) population wintering in Sahel, we investigated potential change in the impact of environmental conditions on survival and recruitment probabilities between 1981 and 2003. 3. We observed a decrease in the strength of the link between survival and Sahel rainfall during the last decade, down to a nondetectable level. Whether Sahel climate was found to affect the survival of storks under droughty conditions, individuals did not seem to respond to climatic variation when precipitation was more abundant. 4. This result gives evidence to a nonlinear response of a migrant bird to wintering environment. Present climate seems to fluctuate within a range of condition providing enough resources to maximize stork's survival. It suggests that whereas inter-annual variability impacted individuals, pluri-annual average condition affected the intensity of this impact. Such pattern may be more widespread than thought, and its modelling will be crucial to predict the impact of future climate change on population dynamics.