Radiotelemetry locates wintering grounds of DDE-contaminated black-crowned night-herons

In a previous study (1978-1980) of blackcrowned night-herons (Nycticorax nycticorax) nesting in the Pacific Northwest (Henny et al. 1984), some exceptionally high levels of DDE were found in eggs. Egg DDE levels >8 ppm (wet weight) were accompanied by low productivity and a high incidence of cracked eggs. A strong north-south clinal pattern of DDE residues in eggs was found, with the southern colonies most contaminated. Observed productivity was below population maintenance at Ruby Lake, Nevada. No breeding ground DDT-DDE contamination in prey species was found except along the Columbia River, where higher than expected (based on latitude relationship only) nightheron egg residues were documented. Fortunately, DDE residues in western night-heron eggs decreased in recent years (through 1983), which resulted in an improved outlook for western populations (Henny et al. In press). The original study left 2 questions unanswered: (1) where did the night-herons obtain the DDE found in their eggs, and (2) what phenomenon explains the geographical cline in DDE residues? Disjunct wintering ranges with slight overlap could explain the observed residue dine if DDE presence was different among wintering localities. Some winter (NovFeb) band recoveries were available at the time of the original study, but the data were insufficient to draw firm conclusions. To answer the questions, we attached radio transmitters to 15 black-crowned night-herons at Ruby Lake and to 14 night-herons at Pocatello, Idaho, to determine wintering localities. In 1979, the Ruby Lake colony was heavily contaminated with DDE (geometric x in eggs = 8.2 ppm [Henny et al. 1984]), while colonies near Pocatello (about 400 km north-northeast of Ruby Lake) contained substantially less (Mud Lake-2.2 ppm, Blackfoot Reservoir-2.4 ppm, Minidoka-3.5 ppm [Findholt and Trost 1985]). Our study was designed to see if there were differences in wintering localities that could explain the 3-fold difference in DDE residues in eggs from the 2 colonies.