Phenological and geographical shifts have interactive effects on migratory bird populations

For many taxa, ranges are shifting toward the poles and the timing of seasonal reproduction is advancing in response to climate change. For migratory birds, changes such as these could produce particularly strong impacts because of their potential to affect migratory timing and distance. Due to the relatively complex life histories of migratory species, however, it is difficult to intuit exactly what these impacts will be. Here, we develop a general population model for a long-distance migrant, introducing a framework for understanding the potential implications of changes in both phenology and migratory distance for bird abundances. We find that population sizes may increase with either shorter or longer migratory distances, depending on the nature of any concurrent phenological changes. This interaction between timing and distance suggests a need to consider multiple potential responses to climate change simultaneously in order to understand the overall impact of climate change on migratory populations. Our results reveal a degree of variability in the qualitative nature of this phenology-distance interaction, suggesting a possible explanation for observed variation in how migratory birds have already responded to climate change.