The exposure of non-target wildlife to rodenticides, with special reference to the red kite (Milvus milvus)
Factors affecting exposure of wildlife to rodenticides were studied in the field. Time lapse video monitoring demonstrated changes in rat behaviour during normal routine rat control. Rats suffering pre-lethal anticoagulant toxicosis lost their thigmotactic behaviour, becoming increasingly active in open space. Poisoned rats also sat motionless in open space for relatively long periods of time and exhibited lethargic and uncoordinated movement. The observed behavioural change is likely to increase the exposure and vulnerability of rodenticide contaminated rats to their predators. Rat carcasses disappeared rapidly where local scavengers were active. Video monitoring showed several scavenging mammals and birds taking carcasses. The red kite is potentially the avian species most at risk of secondary poisoning by rodenticides, owing to its scavenging niche and foraging around farms. Observations of captive adult birds highlighted a preference for viscera of rats, which may increase the likelihood of rodenticide exposure if rats were poisoned. Monitoring of parental provisioning of prey items to nestlings showed rats to be a major component of diet. Hatchlings were fed only the viscera, indicating that nestlings of this age may be at greatest risk of secondary poisoning. Study of non-target small mammals as route of exposure revealed that a large proportion of local woodmouse and vole populations fed from bait stations during normal rat control. Residue analysis demonstrated substantial body burdens that may be transferred to small mammal consumers. Calculations indicated that some predators might be a high risk of receiving a lethal dose if they foraged in a rodenticide treated site and consumed poisoned non-target small mammals.