Partial migration, whereby some individuals migrate and some do not, is relatively common and widespread among animals. Switching between migration tactics (from migratory to resident or vice versa) occurs at individual and population levels. Here, we describe for the first time the movement ecology of the largest wintering population of Egyptian Vultures (Neophron percnopterus) in south-west Europe. We combined field surveys and GPS tracking data from December to February during four wintering seasons (2014–2018). The wintering population consisted on average of 85 individuals (range 58–121; 76% adults and 24% subadults). Individuals were counted at five different roosting sites located near farms, unauthorized carcass deposition sites and authorized carcass deposition sites. Our results show that vultures tend to remain close to the roosting site. Moreover, we observed that females exhibited smaller home range sizes than males, which suggests a possible differential use of food sources. Overall, birds relied more on farms than other available food resources, particularly subadult individuals which exploited more intensively these sites. Our results showed that Egyptian Vultures congregate in significant numbers at specific sites throughout the winter period in south-west Spain and that these roosting and feeding sites should be given some level of legal protection and regular monitoring. Furthermore, predictable food sources might be driving the apparent increase in the non-migratory population of Egyptian Vultures, as observed in other avian species which are also changing their migratory behavior.

Partial migration is one of the most widespread migratory strategies among taxa. Investigating the tradeoff between environmental/social factors -tness and energetic consequences -is essential to understand the coexistence of migratory and resident behaviours. Here, we compiled eld 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 tness 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 ight e ciency, 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 ew more e ciently at longer distances, bene tting 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 bene t 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 bene t future breeding. Our results offer new insights to understand how species bene t from one strategy or another and that the coexistence of both migratory forms is context-dependent.

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.