Lethal Mobilization of DDT by Cowbirds

Abstract

This study is an experimental demonstration of lethal mobilization of DDT by brown-headed cowbirds (Molothrus ater) and the effects of food deprivation on the distribution and loss of DDT, DDD, and DDE. The principal experimental group consisted of 20 birds fed a dietary dosage of 100 ppm of DDT for 13 days. After 2 days of full rations of untreated food, they were subjected to food restriction. Food was reduced to 43 percent of normal. Seven of the 20 birds died within 4 days. No birds died in the three control groups, treated as follows: ( 1 ) 20 birds fed 100 ppm DDT for 13 days and full rations of untreated food thereafter, (2) 20 birds fed only untreated food but subjected to food restriction, and (3) 20 birds fed full rations of untreated food throughout. In a pilot study, birds were fed 100, 200, or 300 ppm of DDT and subjected to two periods of food restriction, the first of these immediately after dosage ceased and the second 4 months later. DDT-dosed birds from all dosage levels died in each period of food restriction. Before the weight loss that accompanied food restriction, the brains of DDT-dosed birds had concentrations of DDT and DDD that were far below the lethal range. Concentrations increased rapidly to lethal levels. In these birds, DDT in carcasses decreased while DDD increased. DDTdosed birds that died during food restriction lost 16 percent of their total body burden of DDT + DDD + DDE, 21 percent of their weight, and 81 percent of their fat. The DDT-dosed birds that were subjected to food restriction but survived lost a significantly greater proportion of their body burden of residues than similarly dosed birds not subjected to weight loss. Brain levels of DDT and DDD in birds that died during food restriction soon after dosage did not differ significantly from brain levels of birds that died in a period of food restriction 4 months after dosage. Concentrations of DDE were significantly higher in the latter group, although they were lower than concentrations considered to be lethal. In contrast, carcass levels of DDT and DDD were significantly lower, and DDE was only slightly higher, in the birds that died in the second period of food restriction. It is concluded that stored DDT residues present a hazard to birds, which utilize stored fat during periods of stress due to reproduction, cold weather, disease, injury, limited food supply, or migration. DDT [l,l,l-trichloro-2,2-bis(p-chlorophenyl)ethane] has been used extensively for insect control in the United States for many years. Direct mortality of birds has followed recommended treatments in many areas (Dustman and Stickel 1966). Birds that survive treatments that kill a portion of the population will themselves carry high residues of DDT in their bodies (Stickel and Stickel 1969). DDT has become sufficiently widespread in the environment so that residues are present in essentially all wild birds and often cannot be traced to specific treatment programs. Mortality of birds due to mobilization of stored DDT during food restriction was suggested by Bernard (1963:178). Additional evidence of the hazard of stored DDT to birds under starvation or other stress was recorded by Stickel ( 1965 ), Ecobichon and Saschenbrecker ( 1969 ), and Stickel and Rhodes (19709. The present study is an experimental demonstration of lethal mobilization of DDT by cowbirds and the effects of food deprivation on the distribution and loss of DDT, DDD, and DDE. We thank W. H. Stickel for assistance in the experiment and D. Hughes for his carefully conducted chemical analyses and his advice on chemical matters.