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Optimal migration theory is a framework used to evaluate trade-offs associated with migratory strategies. Two strategies frequently considered by migration theory are time minimizing, whereby migration is completed as quickly as possible, and energy minimizing, whereby migration is completed as energetically efficiently as possible. Despite extensive literature dedicated to generating analytical predictions about these migratory strategies, identifying appropriate study systems to empirically test predictions is difficult. Theoretical predictions that compare migratory strategies are qualitative, and empirical tests require that both time-minimizers and energy-minimizers are present in the same population; spring migrating silver-haired (Lasionycteris noctivagans) and hoary bats (Lasiurus cinereus) provide such a system. As both species mate in the fall, spring-migrating males are thought to be energy-minimizers while females benefit from early arrival to summering grounds, and are thought to be time-minimizers. Thermoregulatory expression also varies between species during spring migration, as female silver-haired bats and males of both species use torpor while female hoary bats, which implant embryos earlier, are thought to avoid torpor use which would delay pregnancy. Based on optimal migration theory, we predicted that female silver-haired bats and hoary bats would have increased fuel loads relative to males and the difference between fuel loads of male and female hoary bats would be greater than the difference between male and female silver-haired bats. We also predicted that females of both species would have a greater stopover foraging proclivity and/or assimilate nutrients at a greater rate than males. We then empirically tested our predictions using quantitative magnetic resonance to measure fuel load, δ13C isotope breath signature analysis to assess foraging, and 13C–labeled glycine to provide an indicator of nutrient assimilation rate. Optimal migration theory predictions of fuel load were supported, but field observations did not support the predicted refueling mechanisms, and alternatively suggested a reliance on increased fuel loads via carry-over effects. This research is the first to validate a migration theory prediction in a system of both time and energy minimizers and uses novel methodological approaches to uncover underlying mechanisms of migratory stopover use.

Optimal migration theory has been used for three decades to generate predictions of stopover behavior and understand migration ecology. Yet, to date, there have been no attempts to understand the impacts of thermoregulation on migration theory predictions of stopover behavior. Though most migrants are homeothermic, a diverse group of migrants from bats to hummingbirds and warblers make use of some degree of heterothermy. We consider how thermoregulation influences stopover fuel deposition rates, and thus alters optimal migration theory predictions of stopover behavior using a hypothetical migratory bat as a model organism. We update the analytical models of optimal migration theory by considering scenarios of fixed metabolic rate (the current assumption of optimal migration theory) and three different mass-specific metabolic rates including homeothermy, shallow torpor heterothermy and deep torpor heterothermy. Our results predict that heterotherms will make shorter stopovers, have a decreased departure fuel load, and reduce the overall time and energy costs associated with stopovers relative to homeotherms, highlighting that thermoregulation can drastically influence stopover behavior and ultimately play a critical role in population level patterns of migration.

Migration is energetically expensive and is predicted to drive similar morphological adaptations and physiological trade‐offs in migratory bats and birds. Previous studies suggest that fixed traits like wing morphology vary among species and individuals according to selective pressures on flight, while immune defences can vary flexibly within individuals as energy is variably reallocated throughout the year. We assessed intraspecific variation in wing morphology and immune function in silver‐haired bats Lasionycteris noctivagans, a species that follows both partial and differential migration patterns. We hypothesized that if bats experience energy constraints associated with migration, then wing morphology and immune function should vary based on migratory tendency (sedentary or migratory) and migration distance. We predicted that long‐distance migrants would have reduced immune function and more migration‐adapted wing shapes compared to resident or short‐distance migrating bats. We estimated breeding latitude of spring migrants using stable hydrogen isotope techniques. Our sample consisted primarily of male bats, which we categorized as residents, long‐distance northern migrants, short‐distance northern migrants and southern migrants (apparent breeding location south of capture site). Controlling for individual condition and capture date, we related wing characteristics and immune indices among groups. Some, but not all, aspects of wing form and immune function varied between migrants and residents. Long‐distance northern migrants had larger wings than short‐distance northern migrants and lower wing loading than southern migrants. Compared with resident bats, short‐distance northern migrants had reduced IgG while southern migrants had heightened neutrophils and neutrophil‐to‐lymphocyte ratios. Body fat, aspect ratio, wing tip shape and bacteria killing ability did not vary with migration status or distance. In general, male silver‐haired bats do not appear to mediate migration costs by substantially downregulating immune defences or to be under stronger selection for wing forms adapted for fast, energy‐efficient flight. Such phenotypic changes may be more adaptive for female silver‐haired bats, which migrate farther and are more constrained by time in spring than males. Adaptations for aerial hawking and the use of heterothermy by migrating bats may also reduce the energetic cost of migration and the need for more substantial morphological and physiological trade‐offs.

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