Avian seed dispersal function is age dependent

Animal species provide important ecosystem functions. However, how individuals within species differ in their contribution to ecosystem functions remains little explored. We investigated seed dispersal movements of 72 juvenile and adult spotted nutcrackers (Nucifraga caryocatactes), which are the sole seed dispersers of Swiss stone pine (Pinus cembra). We compared seed dispersal ranges, variability in hourly displacement, elevation of occurrence and the preferred seed caching elevation between juvenile and adult birds. Overall, juveniles had significantly larger seed dispersal ranges, higher variability in displacement and occurred and cached seeds at higher elevations than adults. Additionally, we investigated a subsample of nine individuals tracked over their first (juvenile) and their second (adult) year. From their first to their second year, birds significantly reduced their seed dispersal ranges but maintained similar hourly displacements and elevational occurrence, indicating ontogenetic shifts in their seed dispersal function. Our results suggest that juvenile spotted nutcrackers are of particular importance for pine seed dispersal, as they disperse seeds over large distances and occupy elevations that are favourable for pine regeneration under changing climate. Our study highlights the importance of age-effects when assessing the contribution of animals to ecosystem functions in current and future environments.