The toxicity of three second‐generation rodenticides to Barn Owls (Tyto alba Scop.) has been investigated. Brodifacoum, difenacoum and flocoumafen were separately fed to owls over a period of 15 days via rodenticide‐fed mice to simulate the potential route of exposure in the wild. The owls survived a cumulative dose of each rodenticide of at least 1.9 mg kg−1 owl body weight over 15 days. This is equivalent to the consumption of two 25 g mice with a rodenticide residue of 1 mg kg−1 each day for 15 days. Residue analysis confirmed that the liver is the organ which retains the largest residue of ingested rodenticide.

Chemical analysis of rodenticide residues in regurgitated owl pellets has been shown to be a sensitive, non‐invasive method for monitoring the potential exposure of Barn Owls to second‐generation rodenticides in their prey. The method, originally developed for flocoumafen, has been extended to two other rodenticides. brodifacoum and difenacoum. The method was validated as part of a toxicity study in which Barn Owls were separately fed 40–130 μg day −1 of brodifacoum, difenacoum and flocoumafen in mice over 15 days. Each day an average of 25% of the consumed rodenticide was regurgitated in the pellets.

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A study was carried out to assess the feasibility of monitoring the exposure of barn owls (Tyto alba) to an anticoagulant rodenticide, flocoumafen, by analysis of residues in regurgitated pellets following consumption of flocoumafen‐contaminated mice. Mice were fed on a diet containing [14C]flocoumafen, equivalent to 12 mg kg−1, and killed 24 h later. A single [14C]flocoumafen‐contaminated mouse was fed to each of four captive barn owls, equivalent to 0·11‐0·23 mg kg−1 per bird, followed on seven successive days by control diet (i.e. undosed mice). The [14C]flocoumafen dose was eliminated by the owls over the eight‐day period in pellets (44%, range 35–55%) and faeces (18%, range 11–21%), with the highest residues being observed in samples from the first 24‐h period. Further detailed analysis of the pellets confirmed that flocoumafen residues in the first‐day pellets represented 15% (range 8–26%) of the original flocoumafen residues ingested by the barn owls. Calculations based on these data and typical flocoumafen residues in live captured rodents (following baiting) confirm that pellet residue analysis is a sensitive and appropriate method for the non‐invasive monitoring of exposure of barn owls to flocoumafen. There were no symptoms of anticoagulant poisoning in any of the birds; two of the birds were successfully paired the next season and produced fledgelings.

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