Swiss birds of prey widely contaminated with rodenticides

The Ecotox Centre has analysed Swiss birds of prey for residues of rodenticides. The results are worrying: 92% of the buzzards and kestrels examined were contaminated with these persistent substances. Whilst direct deaths from poisoning were rare, the high level of sublethal contamination raises questions about the long-term health of our native birds of prey.

In modern pest control, rodenticides from the anticoagulant group are the treatment of choice against rats and mice. They block the vitamin K cycle in the liver, which prevents blood clotting. The rodents die a few days after ingestion from internal bleeding. However, this delayed death poses an ecological problem: the poisoned animals become easy prey for birds of prey, which thus ingest the poison with their food and accumulate it in their own livers.

A widespread phenomenon

In a pilot study conducted several years ago, the Ecotox Centre had already established that native wildlife is often contaminated with rodenticides. To assess the extent of contamination in Switzerland more accurately, researchers at the Ecotox Centre, on behalf of the Federal Office for the Environment and in collaboration with the Vetsuisse Faculty of the Universities of Zurich and Bern, have now examined 103 carcasses of native buzzards and kestrels. The birds were provided by the Berg am Irchel Bird of Prey Centre, the Sempach Ornithological Station and the Landshut Wildlife Centre.

The analyses of the liver samples, carried out using high-performance liquid chromatography, revealed a clear picture: 92% of the birds examined were contaminated with rodenticides from the anticoagulant group. Concentrations ranged from minimal traces to higher levels of 582 ng/g. “Due to their chemical properties, these substances degrade very slowly and can accumulate in the animals’ tissues,” explains study leader Sibylle Maletz.

Dangerous sublethal effects

A key question addressed by the study was the clinical relevance of these residues. Researchers from the Vetsuisse Faculty therefore determined the most likely cause of death in the animals through macroscopic examinations. In only around 4% of the animals was internal bleeding, indicative of rodenticide poisoning, suspected to be the cause of death. The majority of the birds died from causes such as exhaustion and lack of food in winter, as well as infections or trauma resulting from collisions.

“But one cannot assume that the exposure is therefore harmless,” says Sibylle Maletz. Almost half of the birds examined (46%) showed concentrations of over 10 ng/g in their livers. In recent studies from North America, this value is considered the threshold above which negative effects are possible: impaired blood clotting can lead to minor injuries healing more slowly. The birds may become more lethargic or lose their coordination. This could lead to premature death: a slightly dazed buzzard is more likely to overlook an approaching car or fail to catch its prey. “The study suggests that exposure to anticoagulants could act as an additional stress factor for the birds,” says Sibylle Maletz. “It could thus reduce the birds’ resilience to natural hazards.”

Monitoring to be expanded

The next steps are already planned to better understand the ecological risk: future studies will increasingly focus on specialised hunters such as barn owls and golden eagles. In addition, methods are being refined to measure exposure using blood samples from live animals. This would allow researchers not only to analyse carcasses but also to monitor anticoagulant exposure and the health status of entire populations in real time.

As birds of prey are often additionally contaminated with heavy metals or pesticides, the combination of these pollutants with anticoagulants could put further pressure on the populations of endangered species. The Ecotox Centre therefore recommends stepping up efforts to reduce anticoagulant inputs and driving forward the transition to less problematic alternatives, such as mechanical traps or less persistent active ingredients.