Recovery of a sparrowhawk population in relation to declining pesticide contamination.
Sparrowhawks (Accipiter nisus) found dead in eastern England showed a significant reduction in HEOD (organochlorine pesticide residues derived from aldrin and dieldrin) concn from about 1975 (following the withdrawal of aldrin and dieldrin from use on autumn-sown cereals), and a reduction in DDE (derived from DDT) concn from about 1980. In a survey in a 220-km2 study area, sparrowhawks began to nest in 1979, after an absence of about 20 years. From 3 nests with an average of 1.8 young per clutch found in that year, numbers increased to 84 nests with an average of 2.9 young per clutch in 1989. The improvement in clutch size was due mainly to an increase in hatching success, in turn associated with improved egg-shell thickness and reduced egg breakage. In 1980-89, HEOD concn in eggs and birds were small and DDE concn declined in both eggs and birds. It is concluded that the recovery of the bird 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 the population but in later years the loss of this pressure, and the associated increase in the reproductive rate, may have assisted the population increase.