Theoretical and empirical investigation of the influence of heterothermy on bat migration ecology

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2020-05

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Abstract

Migration is one of the most conspicuous natural phenomena on the planet yet remains one of the most difficult to study and least understood. Faced with natural history gaps, optimal migration theory has been used for 30 years as a way to theoretically understand migration and continue to propel the field of migration ecology forward. Migration theory models have been used to generate testable predictions of stopover use, flight dynamics, and nearly every other aspect of migratory behavior. Yet, from inception, optimal migration theory assumed migrants to all be homeothermic, excluding a wide range of heterothermic birds and bats. Thus, I use the question, ‘how does heterothermy influence optimal migration theory predictions of migratory behavior?’ as the starting point of this dissertation.

I first incorporated considerations of heterothermy (torpor-assisted migration) into optimal migration theory to determine how classic analytical predictions of stopover differ between migrants with varied thermoregulatory capacities. The resulting models predicted heterothermic migrants to have reduced fuel loads and shorter duration stopovers relative to homeothermic migrants. Next, I empirically tested optimal migration theory predictions related to differences in migration strategy (i.e., time-minimizing and energy-minimizing) and thermoregulatory expression (i.e., heterotherm and homeotherm), in a system of spring migrating silver-haired bats (Lasionycteris noctivagans) and hoary bats (Lasiurus cinereus). Field methods included quantitative magnetic resonance to measure body composition, and δ13C isotope breath signature analysis to measure foraging proclivity and an index of nutrient assimilation. The revised optimal migration theory predictions of fuel load were supported, but field observations did not support the predicted refueling mechanisms, leading to new interpretations of migratory behavior. Next, I resolved questions related to trade-offs associated with foraging during short duration stopovers observed in migratory bats. I used energy budget models to predict the prey abundance and stopover arrival time required to offset stopover existence costs and recoup fuel deficits via foraging, and then empirically tested these predictions to determine if silver-haired bats arrive to stopover with enough time to achieve energy balance prior to sunrise. I found that two-thirds of migrants were captured at stopover prior to the arrival threshold suggesting that most individuals are able to offset stopover energy costs via foraging prior to sunrise. Finally, in light of the complexity associated with bat migration, I proposed and developed a novel framework that connects population patterns of migration to individual behaviors. Using this framework, I generated more holistic descriptions of migration patterns, in a way that will help guide conservation practitioners, prioritize research trajectories, and facilitate the production of working hypotheses of migration, with an emphasis on explicit assumptions. Incorporating considerations of thermoregulation into optimal migration theory leads us to a new understanding of how migrants may cope with environmental constraints and results in a diversity of individual migratory expressions leading to complex patterns of migration.

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Keywords

Migration, Bat ecology

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