Gathering demographic information on rare species is critical to understanding their population dynamics and implementing efficient conservation measures. Using integrated models, we jointly analyzed multiple data sets, including capture–recapture, GPS tracking and nest monitoring data collected over the last 10 years, to provide the first demographic insights for one of the world's rarest raptors, the endemic Reunion harrier Circus maillardi . We estimated key demographic rates including annual survival and breeding parameters (clutch size, hatching and fledging success), and used population projection models to assess population growth rate and quasi‐extinction risk. In order to guide future conservation actions for the population, we evaluated the effects of different management scenarios that improve survival, fecundity, or both, on population growth and quasi‐extinction risk. Comparison of the estimated annual survival (juvenile and subadult survival: 0.66; adult survival: 0.71) and breeding parameters (clutch size: 2.3; hatching success: 0.45; fledging success: 0.83) with those of other harrier species suggests that adult survival and breeding parameters of Reunion harriers are low. A small data set collected 40 years ago suggests that the probability of an egg producing a fledgling was higher and has declined to the current low level. The population models project that the Reunion harrier population is declining and faces a high risk of quasi‐extinction in the next 40 years. Only management measures that simultaneously improve adult survival and fecundity could lead to a recovery of the population. These alarming results call for immediate conservation action aimed at rapidly improving the demographic rates.