EFFECTS OF CYCLODIENE INSECTICIDES ON THE SPARROWHAWK (ACCIPITER NISUS) IN BRITAIN - A REAPPRAISAL OF THE EVIDENCE

Cyclodiene insecticides were introduced in Britain in the mid-1950s, a fter which there was a rapid decline of sparrowhawk populations in agr icultural 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 dressing s in 1975. Frequency distribution diagrams of dieldrin residues in spa rrowhawk livers were constructed for different geographical areas over specified periods, During 1963-1975, biphasic distributions were foun d, with some 50% of the entire sample contained within a second peak o f geometric mean 20 p.p.m. in an eastern area where cylodiene exposure was greatest and populations were most depressed. A similar second pe ak was found in a sample from another area where the cyclodiene exposu re was less, although it only represented 5% of the individuals found dead. These individuals with high dieldrin levels had evidently receiv ed lethal doses of insecticide. In a sample from the eastern area take n during 1976-1982, the peak at 20 p.p.m. had virtually disappeared, t o be replaced by another peak centred on 4.8 p.p.m. This latter peak m ay have represented individuals dying as a consequence of sublethal ne urotoxic effects. The distribution diagrams for 1983-1986 showed singl e peaks of dieldrin centring on 0.55-0.75 p.p.m., with no clear eviden ce of other peaks which might represent poisoned individuals. By this time, the populations in both areas had either fully recovered or were rapidly recovering.