Carry-over effects from passage regions are more important than breeding climate in determining the breeding phenology and performance of three avian migrants of conservation concern

Long distance migrants are declining more rapidly than residents, with birds that breed in Europe and winter in tropical Africa providing particularly clear examples. Causal mechanisms may include climate change, but are poorly understood partly because carry-over effects from non-breeding ranges can influence breeding performance. Using long-term data spanning four decades we assess how climatic variation in migrants’ winter, passage and breeding ranges determine timing of breeding and reproductive success. We do so for three Afro-European avian migrants of regional conservation concern (redstart, spotted flycatcher and wood warbler). We find that carry-over effects from passage regions consistently had stronger impacts on breeding phenology than breeding climate. Warm Mediterranean passage conditions promoted earlier breeding in all species, and redstarts also bred earlier following higher Sahel rainfall. Warmer springs on the breeding grounds promoted slightly earlier breeding in redstart and wood warbler, but not spotted flycatcher. Carry-over effects also typically influenced breeding performance to a greater extent than weather on the breeding grounds. Greater rainfall in the Sahel increased redstart brood size, warmer Mediterranean passage conditions increased spotted flycatcher brood size and, to a lesser extent, the number of wood warbler fledglings. In contrast to the concern regarding climate change impacts on migrants’ breeding grounds we found no evidence that warmer temperatures on the breeding grounds were associated with reduced reproductive performance. We thus find that climatic variation on the non-breeding grounds, especially passage regions, typically influenced migrants’ breeding phenology and demography more strongly than equivalent variation on the breeding sites. Such carry-over effects should be considered when assessing the causes of migrants’ marked population declines.