Annual variation in long-distance dispersal driven by breeding and non-breeding season climatic conditions in a migratory bird

Long-distance dispersal is a fundamental process in ecology and evolution but the factors that influence these movements remain poorly understood in most species. We used stable hydrogen isotopes to quantify the rate and direction of long-distance immigration in a breeding population of American redstarts and to test whether the settlement decisions that result in long-distance dispersal are driven by habitat saturation or by the phenology of breeding-season resources. Our results provide evidence that both natal dispersal and breeding dispersal were influenced by the timing of breeding-season phenology, with both age classes more likely to disperse north in years when the onset of breeding-season phenology occurs earlier than normal. Yearlings were also more likely to disperse north following winters with poor habitat quality on their non-breeding grounds, demonstrating that carry-over effects from the non-breeding season influence natal dispersal in this species. Collectively, these results are consistent with the hypothesis that American redstarts use the phenology of breeding season resources as a cue to select breeding sites. Our results suggest that long-distance dispersal may allow individuals to rapidly respond to advancing phenology caused by global climate change, though their ability to do so may be constrained by long-term decline in habitat quality predicted for their tropical non-breeding grounds.