Exploring juvenile golden eagles' dispersal movements at two different temporal scales

Abstract

The spatial distribution of populations is the result of individuals' movements. In territorial species, the spatial dynamics of populations is to a large degree shaped by individuals' ranging behaviour during their juvenile dispersal. Here we use information on juvenile golden eagles (Aquila chrysaetos) tracked by satellite telemetry, to explore the effects of daily behavioural decisions on their dispersal strategy during their first year of life. When analysed on a biweekly basis, the large-scale movements of golden eagles did not differ from a correlated random walk (CRW) model of dispersal. Although in the long term such a strategy maximises the acquisition of information on good hunting areas, finding a mate and a vacant territory in a landscape, it is expensive in terms of energy requirements. At the finer scale of daily foraging movements, movement patterns were non-random, probably reflecting animals' oriented movements towards sites with high chances of successful hunting. We suggest that the key issue to take into account when exploring the factors determining individuals' dispersal strategies is how strongly the daily movements are influenced by an active search for food. This is to a large degree determined by food availability and individuals' ability to accumulate reserves. In our case, the individuals with the lowest proportion of foraging habitats within their dispersal areas showed large-scale movements more restricted than expected from a CRW. Also, distances covered by individuals in their biweekly movements were both larger and more variable in winter, when food availability diminishes.