White-Faced Ibis Populations and Pollutants in Texas, 1969-1976

Abstract

Eggshell thickness, levels of pollutant residues, and population status of the white-faced ibis (Plegadis chihi) were monitored in Texas from 1969 through 1976. Texas ibis nesting populations declined by 42%. Reproductive success apparently was limited by DDE-induced shell thinning and by dieldrin-caused mortality. Eggshells averaged 4% to 10% thinner than the pre-1943 mean thickness. Shells of numerous crushed eggs exceeded 20% thinning. Mean DDE residues in randomly collected eggs decreased from 0.94 ppm in 1970 to 0.25 ppm in 1976. DDE was higher, averaging 2.5 ppm, in thin-shelled eggs. DDE was negatively correlated with shell thickness. Dieldrin and PCB residues increased slightly from 1970 to 1976, but neither was correlated significantly with shell thickness. Residues in brains of many dead and dying ibises collected during years of extreme reproductive failure, 1970 and 1973, contained between 5 and 25 ppm dieldrin. Ibises were exposed to aldrin and dieldrin by feeding in rice fields where aldrin-treated rice seed was planted. Improved nesting success in 1976 may have been related to declining residues of DDE and to the discontinued use of aldrin in rice fields. Eggshell thinning has been detected in a number of declining species of fish-eating birds in the United States (Hickey and Anderson 1968, Blus 1970, Anderson and Hickey 1972, Blus et al. 1979). Laboratory investigations have shown that DDT or its metabolites, primarily DDE, induced shell thinning in susceptible species at concentrations found in some wild birds and their eggs (Heath et al. 1969, Wiemeyer and Porter 1970, Haegele and Hudson 1973). The recent population decline in the brown pelican (Pelecanus occidentalis) and the apparent decline in the white-faced ibis (Plegadis chihi) on the Texas Gulf Coast (King et al. 1977, 1978) prompted us to study the extent of eggshell thinning and the impact of organochlorine and heavy metal contamination on these species. The present paper presents information on eggshell thickness changes and chemical resi-