The effects of orchard pesticide applications on breeding robins

Abstract

From 1966 through 1968, robins reproduced successfully in commercial apple orchards which were periodically sprayed with DDT, dieldrin, and other pesticides. Observations by a Z-man team using walkie-talkies revealed that breeding robins obtained essentially all food for themselves and nestlings from unsprayed areas adjacent to the orchards. Invertebrate trapping in sprayed and unsprayed areas showed that these food items were 5 or 6 times more abundant in unsprayed habitat. Worms forced to live in sprayed orchard soil displayed significantly greater mortality than controls. Mean robin clutch sizes in the study orchards were lower than those reported for robins in other studies, perhaps because of food shortage and/or increased foraging distances. Levels of DDT and its analogs in food items from robin foraging areas did not exceed 8 ppm wet weight basis. From late April to July, adult robins showed small but significant increases in DDE levels in all tissues examined, as well as an increase in dieldrin in brains. Pesticides sprayed on the farm had no direct demonstrable adverse effects on the robins; productivity was high and adult mortality low. The situation was in large measure fortuitous, since any changes in orchard management practices which resulted in the presence or availability of invertebrates under orchard trees would be expected to result in robin mortality and/or reduced breeding success.