The Impact of Pesticides on the White-Faced Ibis

The white-faced ibis (Plegadis chihi) which nested in northern Utah was assessed as a species with a relatively low reproductive potential. Ibises normally laid either three or four eggs in a clutch and did not renest persistently, nor with good success, if initial nesting attempts failed. Evidence indicated that the birds did not breed until at least 2 years old. Competition for food, exposure to severe weather, and predation were conspicuous sources of nestling mortality. White-faced ibises usually fed in irrigated agricultural areas, where they came into direct contact with insecticides, such as DDT which was used routinely in northern Utah until 1971. The most frequent food items of the ibises were insect larvae and earthworms, hence the birds were subject to food-chain concentrations of pesticide residues. White-faced ibises commonly laid eggs with cracked or broken shells from 1968 through 1971, but the incidence of aberrant eggs decreased after 1971. By 1974, no significant difference was found between the current mean eggshell thickness and the thickness of eggshells collected before 1940 and preserved in museums. Also, in 1974, less than 1 percent of nests surveyed contained cracked eggs. White-faced ibises in Utah again laid thin-shelled eggs in 1975 and 1976, Cracked eggs were found in about 30 percent of nests examined during both of these years, and the means of eggshell thickness were significantly less than in 1974. A high incidence of cracked eggs was associated with less than 10 percent thinning of eggshells. Eggshell thickness of white-faced ibis eggs collected in 1975 was significantly related, linearly and negatively, to the logarithms of DDE residues of the eggs. The comparison of this relationship to those of other species indicated that the white-faced ibis is especially sensitive to eggshell thinning. DDE residues were found in samples of blood serum, breast muscle. and subcutaneous fat of white-faced ibises collected in 1974 and 1975. The logarithm of DOE in blood serum was positively correlated with 1n DDE in fat and with ln DDE in muscle. DDE levels in blood serum were related to lipid mobilization and varied by season and between sexes