DDT and dieldrin: effects on resident Peregrine Falcon populations in south-eastern Australia

Organochlorine residues in eggs and prey species of the Peregrine Falcon,Falco peregrinus, were determined in parts of three south-eastern Australian States: Tasmania (1975-91), Victoria (1975-83) and South Australia (1977-1981). Relationships between residues detected and eggshell thickness, nest site occupancy and productivity were examined. Temporal changes and, in particular, the relative effects of DDE and dieldrin were also investigated. DDE was detected in all 137 eggs analysed. The concentration of DDE residues in eggs peaked in the late 1970s to early 1980s in Victoria, but peaked earlier and had declined to low concentrations by the 1980s in Tasmania. Maximum concentrations reached 82 ppm, in Victoria. The geometric means were: Victoria 12.4 ppm; Tasmania 1970s 10.1 ppm, 1980s 1.5 ppm; and South Australia 1.95 ppm. Dieldrin was detected in approximately half the eggs, but all except one egg (1.5 ppm) contained concentrations considered noncritical (<1 ppm). The degree of shell-thinning and reproductive success were related to DDE content, but not to that of dieldrin.Organochlorine concentrations in prey reflected general concentrations in eggs and, with a few exceptions, were low. In Tasmania, reproductive success increased as DDE contamination declined. In Victoria, despite mean concentrations of DDE well above that considered to be critical for successful reproduction (>15-20 ppm) for at least seven years, the breeding population declined by <2% a year. We interpret this result as evidence that DDE causes a slow decline in population and that the dramatic declines observed in some other parts of the world were mainly attributable to contamination by dieldrin or to other confounding factors.