Long-term population monitoring reveals an alarming decline in the endangered endemic Reunion Harrier Circus maillardi

Insular species are disproportionately vulnerable to extinction due to small population sizes, geographic isolation, and high susceptibility to stochastic and anthropogenic pressures. The endemic Reunion Harrier (Circus maillardi), the last breeding raptor on La Reunion (Indian Ocean), is currently listed as "endangered" (EN) on the IUCN Red List. Concerns have recently emerged regarding the demographic impacts of secondary poisoning from anticoagulant rodenticides used for rat control in agricultural landscapes, in addition to other threats such as habitat loss and fragmentation, and reduced genetic diversity. Here, we analysed data from three standardized island-wide breeding censuses (1998-2000, 2009-2010, 2017-2019), encompassing 355 sampling points and totalising > 1,500 h of observations, to quantify overall population trends and spatial variation within those. Using generalized additive mixed models accounting for spatial variation in the different census designs, we estimated a significant decrease of -46% (95% CI: -61% to -28%) in the relative abundance of breeding pairs at the island scale over 21 years (equivalent to three generations), which supports its current endangered (EN) status. Decline was more pronounced in the eastern part of the island (-56%, 95% CI: -69% to -40%) compared to the western part (-29%, 95% CI: -52% to +0.3%). Combined with previous work on inbreeding and mutational load, exposure to anticoagulant rodenticides and ongoing threats (habitat changes), these results suggest this harrier population is undergoing an extinction vortex that requires immediate and targeted, spatially explicit, conservation actions. We also recommend that population size continues to be monitored and that knowledge of demographic parameters be improved to guide adaptive management for the species.