Assessing the implications of global climate change for migratory bird populations has been constrained by seasonal, spatial, and climatic limitations, resulting in incomplete or skewed conclusions. Here, we use a multivariate approach to summarize how passerine bird species that migrate within the Western Hemisphere (n = 77) are associated with projected novel climates. We expect associations to be strongest during the non-breeding season when species occur in the tropics or subtropics where novel climate projections are strongest. We used weekly estimates of relative abundance to delineate species’ distributions, and we considered projections for three climate variables (precipitation rate, minimum and maximum temperature) standardized using historical inter-annual climate variability over a 60 year period. As expected, species on their non-breeding grounds were associated with highly novel climates, and this was more pronounced for species that winter in closer proximity to the tropics (ca. 45-75% of distributions identified as novel). Associations with the least novel climates occurred during the end of spring migration and the beginning of the breeding season (ca. 35%). These associations, however, transitioned back to highly novel climates at the end of the breeding season and the beginning of autumn migration (ca. 80%). This unexpected outcome is a consequence of greater projected warming occurring in the autumn on species’ temperate breeding grounds, a region characterized by low inter-annual climatic variability at this time of year. Thus, species are projected to encounter novel climates across the full annual cycle, with a brief period of lower novelty followed by a pronounced peak in novelty occurring during the transition from breeding to migration. When assessing the implications of climate change for highly mobile taxa such migratory birds, our findings emphasize the importance of examining standardized climate change projections within an all-inclusive spatial and temporal context that covers the full annual cycle.
Frank LaSorte is a Research Scientist at The Cornell Lab of Ornithology. His work focuses on exploring the patterns and dynamics of biological diversity across space and time at continental to global scales. Currently he is developing a collaborative project to examine population-level migration trajectories of North American birds using the eBird database. The goal of this work to provide integrative and comprehensive scientific insights that will inform our current understanding of the biological phenomenon of avian migration. This research project provides unique opportunities to test and refine current migration theory and much needed information to support better informed policy and conservation initiatives.