Forty adult male cowbirds were fed a diet containing 20 ppm dieldrin; 20 of the birds were randomly selected to die from dieldrin poisoning and 20 were sacrificed when dieldrin had made them too sick to eat. An average of 6.8 ppm dieldrin (range of 1.51 to 11.7) in the brain on a wet-weight basis was associated with a treatment-related cessation of feeding, whereas an average of 16.3 ppm (range of 9.84 to 23.5) was found in the brains of birds that died from dieldrin poisoning; the latter concentrations agreed with those determined in other studies. Dieldrin-induced starvation was generally irreversible; therefore, brain levels of dieldrin that are clearly sublethal may nevertheless present a grave hazard to birds by initiating a process that leads to death. Fatter cowbirds were able to survive longer on dieldrin treatment but contained brain residues similar to those in cowbirds that died sooner. Some cowbirds survived for 2 months or longer with unexpectedly large amounts of body fat remaining when they died or were sacrificed. Fatter cowbirds also survived longer after they had stopped eating.
