Effects of Cyclodiene Insecticides on the Sparrowhawk (Accipiter Nisus) in Britain – a Reappraisal of the Evidence

Abstract

Cyclodiene insecticides were introduced in Britain in the mid-1950s, after which there was a rapid decline of sparrowhawk populations in agricultural areas associated with the lethal toxicity of these compounds. Subsequently the recovery of sparrowhawk populations was studied in different areas of Britain following the ban on dieldrin seed dressings in 1975. Frequency distribution diagrams of dieldrin residues in sparrowhawk livers were constructed for different geographical areas over specified periods. During 1963–1975, biphasic distributions were found, with some 50% of the entire sample contained within a second peak of geometric mean 20 p.p.m. in an eastern area where cylodiene exposure was greatest and populations were most depressed. A similar second peak was found in a sample from another area where the cyclodiene exposure was less, although it only represented 5% of the individuals found dead. These individuals with high dieldrin levels had evidently received lethal doses of insecticide. In a sample from the eastern area taken during 1976–1982, the peak at 20 p.p.m. had virtually disappeared, to be replaced by another peak centred on 4.8 p.p.m. This latter peak may have represented individuals dying as a consequence of sublethal neurotoxic effects. The distribution diagrams for 1983–1986 showed single peaks of dieldrin centring on 0.55–0.75 p.p.m., with no clear evidence of other peaks which might represent poisoned individuals. By this time, the populations in both areas had either fully recovered or were rapidly recovering.