TIME IS ENERGY: DRIVERS OF MIGRATORY AND NON-MIGRATORY MOVEMENTS BY SOARING BIRDS
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Vultures are obligate scavengers and search over large areas for carrion, which is ephemeral. To profit from carrion, they are also obligate soarers that rely on the availability of environmental updrafts to subsidize flight. This restricts their flight spatially and temporally to where and when strong updrafts are available. In this dissertation, I investigate how Turkey Vultures (Cathartes aura) use stopovers to both avoid inclement weather and to replenish spent fuel reserves, as well as how the timing of movement activity differs according to flight mode.Using GPS-tracking data from four migratory Turkey Vulture populations, I evaluated how Turkey Vultures respond to changing weather conditions. During stopovers, movement activity was positively correlated with conditions that promote thermal development, suggesting not all stopovers are used for weather-avoidance. Turkey Vultures began stopovers immediately in response to deteriorating weather conditions but their departure from stopovers was delayed relative to improvements in weather, behavior that is consistent with an energy-minimization strategy. I estimated total energetic costs for each migration and identify probable refueling stopovers. Only long-distance Turkey Vulture migrants regularly stop to feed. Overall migratory costs are driven by migration duration, and therefore are lower in the spring when vultures migrate faster, which may contribute to seasonal differences in flight behavior. Last, I compared the non-migratory movements of 49 avian species to test for the influence of flight mode on the timing of movement activity. Terrestrial soaring birds began activity later and stopped activity earlier than other birds. This study demonstrates that flight mode influences temporal patterns of daily movement activity of birds. This dissertation enhances our understanding of how soaring birds cope with the temporal and spatial restrictions on their movements. During non-migration, soaring birds are active for longer proportions of the day to search for food over greater spatial areas than flapping birds. During migration, total energetic costs are driven more by migration duration than by behavior. Therefore, an energy minimization strategy for Turkey Vultures is also a time minimization strategy and vultures are expected to optimize their migration so that they spend the fewest days migrating as possible.