Long-distance dispersal is asymmetrical with respect to age, sex and breeding latitude in a long-lived monogamous bird

Although relatively rare, long-distance dispersal significantly impacts population persistence by facilitating range expansion, range shifts, and genetic exchange. For individuals dispersing northwards, it may be a suitable adaptation strategy to escape negative effects of climate change on their original breeding sites. In this study, we constructed a joint live encounter – dead recovery model under a Bayesian multistate framework to quantify long-distance dispersal between the Barents Sea, Baltic Sea, and North Sea subpopulations of the Russia/Germany & Netherlands flyway population of barnacle goose (Branta leucopsis), using long-term mark-recapture data of 22,413 individuals ringed between 1995 – 2023. Long-distance dispersal was strongly biased by age-, sex- and direction. Natal dispersal predominantly occurred in northward direction with 23.9% of juvenile males and 8.6% of juvenile females estimated to transition annually from the North Sea to the Barents Sea subpopulation. In contrast, breeding dispersal in the same direction in adults was minimal, estimated at only 0.49% and 0.21% for males and females respectively, and was not always distinguishable from temporary (moult-) migrations. Our model results were validated with data from 14 dispersing individuals, 9 of which were male, for whom the timing of breeding or moult was recorded. In all cases, dispersal was in northward direction and timing of breeding or moult of dispersers more closely resembled the timing of the receiving than of the original subpopulation, but more so in males than in females. Our results support the notion of strong male-biased natal dispersal in monogamous waterbirds. Interestingly, despite substantial growth in the temperate breeding subpopulations during our study period, natal dispersal occurred predominantly in northward direction at both individual and population levels. The unidirectional long-distance dispersal observed is expected to result from the unique flyway structure, were subpopulations with large differences in population size mix during wintering. Additionally, we also highlight the adaptability of dispersers, showing that barnacle geese can adaptively switch migration on and off, and that plasticity in timing of breeding and moult may be larger in males than in females. We argue that this could be an additional explanation for the predominantly male-biased northward dispersal observed in barnacle geese.