Westward Ho! Evidence of Longitudinal Migration in Silver-haired Bats from Hydrogen, Carbon, and Nitrogen Stable Isotopes

Migration is an energetically demanding process and North America’s migratory bats are facing the additional pressure of mortality from rapidly expanding wind energy facilities. Knowledge of bat migrations can be used to identify critical habitat and direct siting and mitigation measures to reduce the impact of wind turbines, but methods to study these movements remain limited. Silver-haired bats (Lasionycteris noctivagans) are a widely distributed bat species in North America that undergoes substantial migrations between their summer and winter locations. Studying these movements has long been a challenge, but technological advances such as the application of stable isotopes to animal migration studies make this more feasible. To date, the majority of the research conducted on silver-haired bat migrations has been focused on their movements east of the Continental Divide. Coastal influences and complex topography have been an impediment to the utilization of stable isotopes in bat migration studies west of this divide. To overcome this challenge, I systematically sampled silver-haired bats for multiple stable isotopes, hydrogen (δ2H), carbon (δ13C), and nitrogen (δ15), across a broad range of geography representative of western North America. Using Generalized Additive Models, I found geographical and climatological correlates of δ2H, δ13C, and δ15N distributions and used these to create the first species-specific stable isotope distribution maps (isoscapes) for silver-haired bats in western North America. I applied these isoscapes in a continuous-surface assignment framework to determine the most-probable migratory origins of silver-haired bats overwintering in coastal regions of southern British Columbia and western Washington State. The stable isotope signatures of these individuals indicated that the majority of them had spent their summers across a broad area further to the east, providing the first empirical evidence of longitudinal migration in silver-haired bats and indicating that coastal areas of British Columbia and Washington State are important overwintering habitat for this species. This work proves that with careful selection of both the samples and stable isotopes used, along with thoughtful consideration of the underlying geographic and climatological processes that drive their ratio distributions, stable isotopes can be used to track seasonal movements of animals in regions of complex topography. These results also emphasize the importance of coastal areas to overwintering silver-haired bats, and the need for strict regulation of wind turbines sited along likely migratory corridors connecting these coastal areas to interior summer habitat.

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