Migration timing responses to climate change differ between adult and juvenile white storks across Western Europe

Abstract

Our goal in this study was to identify age- and population-specific responses to climate change in the autumn migration phenology of a long-lived bird species, the white stork Ciconia ciconia, at the macroscale of its entire migration route in western Europe. We used a 40 yr data series of ring recoveries of adult (>1 yr) and juvenile (<1 yr) white storks to determine the date of autumn passage through the Strait of Gibraltar. We then modelled geographical variability in the timing of autumn migration between age classes across Europe from environmental conditions at the breeding grounds using age-specific generalized additive model (GAM) analysis. Best-fit models accounted for up to 31% of the variability in the data and indicated a progressively earlier passage date over the course of the 40 yr dataset. However, responses varied among populations and age classes. A trend towards earlier migration in juvenile storks was observed, whereas in adults the trend was highly variable between years. In addition, advances in autumn passage dates of juvenile birds were larger in southwestern Europe. Differences between white stork age classes in response to environmental conditions on the breeding grounds are likely caused by different mechanisms for migration adjustment governing adult and young bird behaviour. Overall, plasticity in adapting to new environmental conditions has allowed western European white storks to rapidly respond to, and even benefit from, recent climatic and environmental changes.