Environmental and Social Correlates, and Energetic Consequences of Fitness Maximisation on Different Migratory Behaviours in a Long-lived Scavenger

Abstract Partial migration is one of the most widespread migratory strategies among taxa. Investigating the trade-off between environmental/social factors - fitness and energetic consequences – is essential to understand the coexistence of migratory and resident behaviours. Here, we compiled field monitoring data of wintering population size and telemetry data of 25 migrant and 14 resident Egyptian Vultures Neophron percnopterus to analyse how environmental and social factors modulate resident population size, compare fitness components (i.e., survival and reproduction), and energetic consequences between migratory and resident individuals across wintering and non-wintering seasons. We observed that food availability positively correlated with the wintering population size and that subadult birds increased linearly with censused adult birds. Residents exhibited higher survival probabilities, but lower breeding activity and higher energy expenditure, less flight efficiency, and lower activity due to shorter winter-day lengths. On the contrary, migratory birds showed lower survival but more breeding attempts but spent less energy and flew more efficiently at longer distances, benefitting from longer days in African wintering quarters. These results suggest that anthropogenic food mediated social attraction could modulate population dynamics and promote residency. Food availability may benefit resident individuals enhancing their survival which may offset higher energy expenditure. Migrant birds, on the contrary, may compensate for the higher costs in terms of survival by a reduction in the energy cost, which may benefit future breeding. Our results offer new insights to understand how species benefit from one strategy or another and that the coexistence of both migratory forms is context-dependent.