Experience reduces route selection for conspecifics by the collectively migrating white stork

Abstract

Migration can be an energetically costly behavior with strong fitness consequences in terms of mortality and reproduction.^{1,2,3,4,5,6,7,8,9,10,11} Migrants should select migratory routes to minimize their costs, but both costs and benefits may change with experience.^12,13,14 This raises the question of whether experience changes how individuals select their migratory routes. Here, we investigate the effect of age on route selection criteria in a collectively migrating soaring bird, the white stork (Ciconia ciconia). We perform step-selection analysis on a longitudinal dataset tracking 158 white storks over up to 9 years to quantify how they select their routes based on the social and atmospheric environments and to examine how this selection changes with age. We find clear ontogenetic shifts in route selection criteria. Juveniles choose routes that have good atmospheric conditions and high conspecific densities. Yet, as they gain experience, storks' selection on the availability of social information reduces-after their fifth migration, experienced birds also choose routes with low conspecific densities. Thus, our results suggest that as individuals age, they gradually replace information gleaned from other individuals with information gained from experience, allowing them to shift their migration timing and increasing the timescale at which they select their routes.

Keywords: Ciconia ciconia; autocorrelated kernel density estimate; collective migration; lifetime tracking; movement ecology; ontogeny; step-selection function.