Long-term patterns of breeding range expansion mirror invasion dynamics during recolonisation of a semi-colonial raptor.

Range expansions are driven by a host of eco-evolutionary mechanisms, with consequences for individual fitness, population dynamics and ecosystem function. Processes at the range front are particularly important in determining rates of expansion and resulting distributions. Species recolonisations are a subset of range expansions which have become a global conservation priority, with a key goal to maximise the rate of spatial recovery. However, this is limited by a poor understanding of the underlying patterns expected during recolonisations and their mechanistic drivers. We address this knowledge gap using a unique long-term dataset, comprising nesting records across 54 years of recolonisation by Osprey Pandion haliaetus in Northern Scotland. We investigate spatiotemporal variation in rates of expansion and the role of conspecifics in determining patterns of settlement, including the distribution and density of nests. Spatial expansion in most years was minimal, relying heavily on long-distance dispersal of rare pioneering individuals beyond the breeding range to drive substantial recovery and thus exhibiting a stratified diffusion pattern. The distribution of nests was consequently non-random, with strong clustering into distinct subpopulations. Our findings suggest that recolonisations follow similar patterns of expansion to biological invasions, highlighting opportunities to combine knowledge across two key areas of ecological research. We also emphasise the disproportionate role of male Ospreys in restricting patterns and rates of breeding range expansion under female-biased dispersal. Adapting conservation management accordingly could accelerate species restoration, including wider application of interventions like artificial nest provisioning and translocations to facilitate long-distance dispersal.