© Marcel Burkhardt
Niederberger, M. (2022)
Modeling the migration behavior and exploring the age-structured migratory connectivity of red kites (Milvus milvus) across the species’ global range
Migration is a crucial part of the annual cycle for billions of animals. Understanding migratory connectivity (i.e., how breeding populations redistribute in space over the annual cycle) is essential to illuminate the intricate relationships in how migrating organisms link ecosystems and species across the globe and to inform evidencebased local and global conservation efforts. Most studies investigating migratory connectivity assume that no subgroups exist within populations, i.e., that populations mix randomly. Yet, many populations have sex or age-specific subgroups that differ in their migratory behavior, a phenomenon called differential migration, which is predicted to affect migratory connectivity.
Here, this rarely studied prediction is revised using a GPS movement dataset of unprecedented size and geographic coverage to derive a range-wide assessment of age-structured migration patterns and migratory connectivity in the red kite, a raptor species endemic to Europe. Daily GPS locations of 2440 complete annual cycles representing 1619 red kites were analyzed to identify age-specific migration strategies and links between breeding season and overwintering locations across the species’ global range. By combining a novel continuous space Bayesian regression approach with a recent range-wide abundance map, five key results are derived:
First, calculations revealed that an estimated 54.4% of the total adult population is migrating to the western wintering grounds, while only 1.3% is migrating to the eastern wintering grounds and the remaining 44.4% remained resident throughout the annual cycle. Second, large variations in the migration strategies of individuals were found between breeding sites but also within a breeding population. Third, migration strategies of individuals changed with age, with younger birds generally showing a higher migration probability than adult birds. Importantly, the strength of this age-dependent differential migration varied substantially across the species range. Fourth, as predicted, migratory connectivity varied with age, with young birds exhibiting higher migratory connectivity (0.34) than adults (0.25). Hence, migratory connectivity of red kites was rather low. Fifth, a migration divide was discovered in eastern Europe, with migration distances to western wintering grounds being significantly longer than to eastern wintering grounds.
As one of the first studies, the effects of differential migration on migratory connectivity across an entire species range were empirically documented within this thesis. By analyzing this massive movement data set of one species, new insights into the overwintering and migratory behavior of red kites throughout the species’ global range could be gained, thus helping to enhance the conservation of this bird species.