Organochlorine residues and eggshell measurements for tree swallows Tachycineta bicolor in Colorado

Reported are organochlorine concentrations in breeding female tree swallows, nestlings, and eggs, and measurements if egg size and eggshell thickness. Residues of six organochlorines were detected in adult carcasses, four in eggs, two in brains, and one in nestlings collected from the study area near Denver, Co. DDEe accounted for 91% of total organochlorine residue; varying concentrations of DDT, PCBs, and other Chemicals were also detected. Eggshell thinning was not demonstrated for these birds. Results fail to prove a strong relationship between DDE contamination and survival or reproduction of tree swallows in this region.