The Réunion harriers is an endemic raptor on Réunion Island. Several threats endanger its population, poisoning by rodenticides being considered as the main one currently. No information is available on its exposure to other chemicals notably trace metal elements such as lead (Pb), mercury (Hg), and cadmium (Cd). The Réunion harrier is still victim of poaching nowadays. When shooting is not lethal, animals may carry embedded shot in their body and thus be exposed to toxic level of Pb as demonstrated for other raptors. Moreover, recent monitoring suggests a decrease of its breeding success over time. It is known that Hg and Cd could impair reproduction and disturb embryo development in birds. The aim is to measure metal concentrations in the tissues of 30 carcasses of harrier collected from 2016 to 2021. Lead was analysed in liver and humerus while Hg and Cd were measured in livers. Concentrations were compared to toxicological reference values. Overall, the Reunion harrier was not exposed to toxic levels of Pb or Cd. For Hg, 53% of the individuals have residues higher than the threshold compatible with oxidative stress, 13% having liver concentrations above those compatible with reproduction impairment. A positive correlation was found between the proportion of urban area within 55 km² around a harrier and Hg concentration in liver. We conclude that Hg exposure could be a threat for the Réunion harrier population and recommend monitoring the exposure of the most sensitive stages, i.e., embryos and nestlings, to this metal with non-invasive methods.

The Réunion harriers is an endemic raptor on Réunion Island. Several threats endanger its population, poisoning by rodenticides being considered as the main one currently. No information is available on its exposure to other chemicals notably trace metal elements such as lead (Pb), mercury (Hg), and cadmium (Cd). The Réunion harrier is still victim of poaching nowadays. When shooting is not lethal, animals may carry embedded shot in their body and thus be exposed to toxic level of Pb as demonstrated for other raptors. Moreover, recent monitoring suggests a decrease of its breeding success over time. It is known that Hg and Cd could impair reproduction and disturb embryo development in birds. The aim is to measure metal concentrations in the tissues of 30 carcasses of harrier collected from 2016 to 2021. Lead was analysed in liver and humerus while Hg and Cd were measured in livers. Concentrations were compared to toxicological reference values. Overall, the Reunion harrier was not exposed to toxic levels of Pb or Cd. For Hg, 53% of the individuals have residues higher than the threshold compatible with oxidative stress, 13% having liver concentrations above those compatible with reproduction impairment. A positive correlation was found between the proportion of urban area within 55 km² around a harrier and Hg concentration in liver. We conclude that Hg exposure could be a threat for the Réunion harrier population and recommend monitoring the exposure of the most sensitive stages, i.e., embryos and nestlings, to this metal with non-invasive methods.