Anticoagulant Rodenticides Exposure in Six Nocturnal Raptor Species from Madrid (Spain)

Abstract

Anticoagulant rodenticides (ARs) have been used for rodent control in both agricultural and urban settings, serving as biocides and crop protection products. Among them, second-generation anticoagulant rodenticides (SGARs) are particularly concerning due to their high persistence and bioaccumulative potential. These compounds can spread through the food chain, affecting various non-target species, with nocturnal raptors especially vulnerable as apex predators. These substances act by inhibiting the enzyme vitamin K epoxide reductase, which prevents the activation of blood clotting factors, causing internal and external bleeding and, ultimately, the death of the animal. Even sublethal doses can lead to clinical symptoms such as anorexia, lethargy, and weakness, increasing the risk of trauma or secondary mortality. This study aimed to assess the prevalence of AR exposure in six nocturnal raptor species in the Community of Madrid: Barn Owl (Tyto alba), Tawny Owl (Strix aluco), Eurasian Eagle-Owl (Bubo bubo), Long-eared Owl (Asio otus), Little Owl (Athene noctua), and Eurasian Scops Owl (Otus scops). A total of 164 individuals admitted to the GREFA wildlife rehabilitation center between 2017 and 2024 were analyzed. Liver samples were examined as the target biological matrix after a systematic necropsy and were tested at the Toxicology Service (SERTOX) of the University of Las Palmas de Gran Canaria using UHPLC-MS/MS. Rodenticides were detected in 71.95% of the birds, predominantly SGARs, with bromadiolone being the most common. Additionally, 64.41% of the positive cases had more than one rodenticide detected, with the most frequent combination being brodifacoum and bromadiolone. Most detected concentrations were ≤100 ng/g, although some exceeded established toxicity thresholds. Statistical analyses showed that adult birds had significantly higher exposure. Species feeding mainly on small mammals had higher residue levels, although insectivorous species also exhibited notable exposure. These results are consistent with previous studies from mainland Spain in diurnal raptors; however, as expected, are lower than those reported in the Canary Islands. This study underscores the need for more ecologically sustainable rodent control strategies to minimize the impact on non-target wildlife.