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 envi- ronmental/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 modu- late overwintering immature population size, compare energetic consequences between migratory and resident individuals across wintering and non-wintering seasons and evaluate fitness components (i.e. survival and reproduction) between the two migratory forms. We observed that social attraction may influence the number of overwintering immature individuals, which increased linearly with adult birds surveyed. Residents spent more energy but exhibited higher survival probabilities and lower breeding activity. On the contrary, migratory birds showed lower energy expenditure during winter but also lower survival and more breeding attempts. These results suggest that social attraction may modulate population dynamics and promote residency in immature birds. Resident individuals benefit from enhancing their survival at the expense of higher energy expenditure during winter. 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 more frequent 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.