Pesticide-related Eggshell Thinning in Australian Raptors

Abstract

Eggshell thickness of 32 species of Australian raptors, relative to DDT use, was investigated. Nine species showed a significant reduction in shell thickness since DDT was introduced to agriculture in 1946: Whistling Kite Haliasfur sphenurus; Brahminy Kite Haliastur indus; White-bellied Sea-Eagle Haliaeetus leucogaster; Marsh Harrier Circus aerugrnosus; Collared Sparrowhawk Accipiter cirrhocephalus; Peregrine Falcon Falco peregrlnus; Grey Falcon Falco hypoleucos: Australian Hobby Falco longipennis; and Southern Boobook Ninox novaeseelandiae. Average reductions in thickness ranged from 2% (Collared Sparrowhawk and Australian Hobby) to 10% (Peregrine Falcon). These levels are unlikely to be causing widespread population declines. However, several clutches from these species were so thin (maximum thinning ranged from 15% for the Grey Falcon to 45% for the Whistling Kite) that they were likely to break during incubation. Thus, localised breakage of eggs and reproductive failure has probably occurred. In general, bird-eating raptors and those living in areas of intensive agriculture were worst affected. Ostensibly, DDT consumption declined from a peak in 1973 to none in 1989. Reduction in shell-thickness occurred as early as 1947 and was greatest throughout the 1970s and 1980s. The uncontaminated eggs of owls were thinner than those of other raptors, perhaps partly reflecting the owls' lower absorption of dietary calcium (bone).