Exploring drivers and costs of partial trans‐Saharan migration in juvenile vultures

Partial migration occurs when only a subset of individuals within a population undertakes a migratory journey. The decision to migrate can be influenced by intrinsic traits (e.g. sex or physical condition) as well as by extrinsic factors (e.g. social environment or resource availability). Understanding the relative importance of in‐ and extrinsic factors is crucial, as migration entails substantial risks, including mortality, with significant implications for both evolutionary processes and conservation efforts. The Griffon Vulture Gyps fulvus represents a special case of partial migration, as this behaviour is almost exclusively limited to first‐year individuals. Such a system is rarely available in other species, where migration mixes inexperienced with experienced individuals preventing the isolation of factors driving migratory decisions. Using GPS tracking data from 139 juvenile Griffon Vultures in southwestern Europe (Iberian Peninsula and southern France), we investigate the intrinsic and extrinsic factors influencing the likelihood of migration during the first year of life, and its associated survival costs. We found that the latitude of each individual's birth site emerges as the main driver of the decision to migrate, with no apparent relationship to local resource availability. This spatially driven migratory behaviour was associated with higher migrant mortality rates in migratory individuals compared to non‐migratory ones, despite the already elevated juvenile mortality observed before the first migratory period. Our findings reveal that partial migration may arise from extrinsic factors associated with the natal location and entail significant survival costs, highlighting the need for a deeper examination of its demographic, evolutionary and conservation consequences.