Estimating the rate of poisoning by insecticide-treated seeds in a bird population.
Poisonings of granivorous birds by cereal seed treatments have been reported in the UK, but the true frequency of poisonings is unknown. We aimed to measure the rate of mortality due to poisoning by seed treatments in woodpigeons (Columba palumbus) in an area of East Anglian farmland where the risk from these compounds appeared high. Woodpigeons were fitted with temperature-sensing radio-tags and monitored daily during the winters of 1994/95 and 1995/96. Birds' deaths were detected and attempts made to retrieve carcasses for post-mortem examination including, where possible, analysis for pesticide residues. Ninety-one woodpigeons were monitored. Eleven birds died, but the causes of their deaths were uncertain: one contained a low residue of insecticide and in the other ten cases, no carcass was recovered, so no analysis was possible. Therefore, the number poisoned by pesticides could lie anywhere between zero and eleven. During 1994/95, estimated mortality ranged from 0% to 52%, depending on how many (if any) of the 11 casualties were poisoned. During 1995/96 there were no casualties. Using conservative diagnostic rules for classifying birds as poisoned by OP seed treatments, no link was found between the availability of treated fields in the study area and the rate of poisonings, and there were no significant differences between the two study years. For reasons discussed in the paper, true mortality resulting from exposure to insecticide seed treatments was considered likely to lie in the range 0-5%.