RECOVERY OF A SPARROWHAWK POPULATION IN RELATION TO DECLINING PESTICIDE CONTAMINATION

1. This paper documents the recovery of a sparrowhawk population in eastern England, associated with a reduction in contamination with organochlorine pesticide residues, namely HEOD (derived from aldrin and dieldrin) and DDE (derived from DDT). 2. Sparrowhawks found dead in eastern England showed a significant reduction in HEOD levels in their livers from about 1975, and a reduction in DDE levels from about 1980. The decline in HEOD was especially marked, and followed a major reduction in the agricultural use of aldrin and dieldrin, which were withdrawn in 1975 from use on autumn-sown cereal grains. From then on, increasing numbers of sparrowhawks were seen in the region, as population recovery occurred. 3. In a 220 km2 study area centred on Rockingham Forest, Northamptonshire, sparrowhawks began to nest in 1979, after an absence of about 20 years. From three nests found in that year, numbers increased progressively to 84 nests in 1989. Over the same period, breeding success improved from an average of 1.8 young per clutch in 1980 to 2.9 in 1989. This improvement was due mainly to an increase in hatching success, itself associated with improved egg-shell thickness and reduced egg breakage. Over the period 1980-89, HEOD concentrations in eggs were low (concentrations in birds having declined earlier), but DDE concentrations declined substantially in both birds and eggs. We suggest that recovery of the sparrowhawk population was primarily dependent on declining HEOD contamination, with a resulting improvement in survival. In the early years of population increase, DDE contamination continued to depress reproduction, but in the later years the decline in DDE contamination (and associated rise in reproductive rate) may have assisted the population increase.