RODENTICIDES IN BRITISH BARN OWLS

Out of 145 Barn Owls found dead through accidents (66%), starvation (32%), shooting (2%) and poisoning (< 1%), 10% contained residues of rodenticides, difenacoum or brodifacoum, in their livers. Difenacoum was in the range 0.005-0.106-mu-g g-1 fresh weight, and brodifacoum was in the range 0.019-0.515-mu-g g-1. Minimum levels of detection were about 0.005-mu-g g-1 for both chemicals. Mice fed for 1 day on food containing difenacoum and brodifacoum died after 2-11 days. Within these mice residues were present at greater concentration in the liver than in the rest of the carcass. The mean mass of residue in a whole 35 g mouse was estimated at 10.17-mu-g (range 4.73-20.65-mu-g) for difenacoum and 15.36-mu-g (range 8.07-26.55) for brodifacoum. Such poisoned mice were fed to Barn Owls for successive periods of 1, 3 and 6 days. All six owls fed on difenacoum-dosed mice survived all three treatments, in which up to an estimated 101.7-mu-g of difenacoum was consumed, and the coagulation times of their blood returned to near normal in less than 5-23 days. Four of the six owls fed on brodifacoun-dosed mice died 6-17 days after the 1-day treatment, but the survivors also survived the 3-day and 6-day treatments. Those that died had each eaten 3 mice, with a combined weight of about 105 g and a total brodifacoum content of about 46.07-mu-g, which was equivalent to a dose of 0.150-0.182 mg kg-1 of owl body weight. After death these owls had 0.63-1.25-mu-g g-1 of brodifacoum in their livers. Blood from the survivors would not coagulate at 9 days post-treatment, but did so at 16 days in one bird and between 38 and 78 days in the other. It is concluded that: (1) Barn Owls in Britain are now widely exposed to second-generation rodenticides; (2) not all owls exposed to these chemicals are likely to receive a lethal dose; (3) brodifacoum is more toxic to owls than difenacoum; and (4) while there is yet no evidence that rodenticides have had any appreciable effect on Barn Owl populations in Britain, further monitoring of residue levels and population trends is desirable.