Climate change and the long-term northward shift in the African wintering range of the barn swallow Hirundo rustica

Recent, unprecedentedly rapid climate change has frequently been invoked as the cause of changes in the phenology of bird migration as well as population decline. Birds would be expected to respond to milder climatic conditions at their breeding grounds by reducing the length of their migration. Here, we exploit the largest ringing recovery database available for a long-distance migrant passerine bird, the barn swallow Hirundo rustica, spanning 1912-2008 and including recoveries from sub-Saharan Africa, to show that this species has shifted its wintering grounds northwards at a rate of 3 to 9 km yr(-1). This shift occurred consistently in the 2 geographical clusters of barn swallows that could be identified on the basis of their migratory connectivity and could be-detected after accounting for possible differential changes in recovery probability among geographical areas. Analyses of trends in climatic conditions at the wintering grounds, based on time series of rainfall and temperature anomalies, showed that this northward shift should have caused a progressively larger proportion of barn swallows to winter in drier or warmer areas, i.e. where primary productivity is lower and therefore ecological conditions for wintering are less favourable. This shift, which may have contributed to the general decline in breeding barn swallow populations, may be due to the combined effects of selection for earlier arrival at the breeding grounds because of milder climatic conditions in the breeding areas, and constraints in other stages of the annual life cycle (e. g. timing of the annual moult) that prevent earlier departure from the wintering grounds.