The demography of an Osprey population under natural recolonisation and translocation processes

In a context of global biodiversity loss, studying the processes underlying the recolonisation—whether natural or assisted—of expanding species provides essential insights for the development of effective conservation strategies. Since the 1980s, the Osprey Pandion haliaetus has naturally recolonized mainland central and eastern France, increasing from one breeding pair in 1985 to 83 in 2024. Between 2018 and 2021, a translocation programme was implemented in south‐western France, using as its donor population the one located in central/eastern France. We aimed to assess the viability of the Osprey population in mainland France, comparing demographic dynamics between the area of origin (Orleans region) in central France and the periphery, and analysing the effects of translocation on survival and on the growth of the donor population. Based on chick ringing and reproductive monitoring since 1995, we estimated survival and reproductive parameters. Population viability analysis showed that this population in mainland France was viable over time with an annual growth rate of 1.05. We detected no demographic differences, in terms of either survival or reproduction, between the peripheral area and the core area, suggesting that the carrying capacity of the environment has not yet been reached in either area. The translocation had no significant effect on the survival of the released individuals and the impact on the donor population remained very limited with a difference in population growth rate of only 0.005—equivalent to a 0.5% annual decrease. The retrospective and prospective analyses highlighted the major influence of adult survival on the variance and sensitivity of the population growth rate, identifying this parameter as a priority to maintain for future conservation actions.