Ecology and ecophysiology of autumn migration in Silver-haired bats (𝘓𝘢𝘴𝘪𝘰𝘯𝘺𝘤𝘵𝘦𝘳𝘪𝘴 𝘯𝘰𝘤𝘵𝘪𝘷𝘢𝘨𝘢𝘯𝘴)

Abstract

The Silver-haired Bat undertakes long-distance latitudinal migrations, in which several aspects of its biology are not understood. Energy is an important constraint on migration, and many aspects of bat migration may be considered in the context of selection pressure to conserve energetic stores. Bats can conserve fat stores to fuel migratory flight by reducing energy spent on other activities. Migration consists of alternating periods of nocturnal flight and daytime stopovers wherein bats must select a temporary diurnal roost. I propose an optimization trade-off framework for selection, wherein animals must balance the value of a roost with the cost of securing it as well as its availability on the landscape. Given that bats use torpor-assisted-migration during daytime inactivity to drastically reduce energy expenditure, there may be little benefit to searching for roost sites that are rare on the landscape but provide multiple benefits. Alternatively, energy conservation is a principal concern during migration, and may drive bats to locate specific beneficial structures. To assess this, I caught and tracked bats to their diurnal roost at a stopover site on Long Point, Ontario, Canada, during fall migration in 2023 and 2024, and evaluated tree as well as surrounding habitat characteristics to evaluate selection on a local- and landscape-scale. Bats roosted in trees that were large and present in dense tree patches, taking opportunities to conceal themselves with a variety of features when convenient. However, bats were not selective for other tree or plot features, suggesting that the size of a roost and tree density are characteristics that aid in reducing the cost (energy, time, opportunity, risk) of searching for a roost. The energy required by nocturnal migratory flights is fueled primarily by adipose fat stores, composed of fatty acids that have a trade-off between energy density and the speed at which they are mobilized to meet the demands of sustained activity. Fatty acids can be synthesized and modified by the body, but some fatty acids that are essential for bodily function are only obtainable from the diet. Because dietary inputs vary geographically, fatty acid profiles can also provide a signature indicative of regional variation in source populations. Thus, stored fatty acids can serve as biomarkers for physiological performance and spatiotemporal structure in a migratory population. I analyzed adipose fatty acid profiles from migrating Silver-haired Bats caught at the same stopover site on Long Point, Ontario in autumn of 2024. My goals were 1) to assess whether fatty acid composition varied within the migratory population, 2) whether these patterns suggest performance trade-offs, and 3) whether fatty acid profiles suggest variation in the origin of migrants arriving at a stopover site. Migrating bats had similar profiles with higher proportions of fatty acids with higher mobilization rate, and similar proportions of essential fatty acids. Interestingly, male bats had a unique mechanism for storing excess fat, wherein heavier bats had higher proportions of energy-dense fatty acids. There was also clear temporal clustering within the population, suggesting distinct geographic origins of two waves of migrants arriving early and late in the migration period. Fatty acid profiles, therefore, offer valuable insights into the physiological strategies and spatiotemporal dynamics of migratory bats. My work highlighted the importance of integrative migration biology, considering multiple aspects of biology across disciplines to inform a holistic understanding of the phenomenon. As well, the use of tree roosts at a common stopover site by several populations of an endangered species with critical energy demands highlights the importance of large-scale ecosystem conservation.