Potential hazard to eastern screech‐owls and other raptors of brodifacoum bait used for vole control in orchards

Abstract

AbstractDuring the fall and winter of 1981–82, a study was conducted to evaluate the secondary poisoning hazards associated with a proposed anticoagulant rodenticide, Volid® (10 ppm brodifacoum), when used to control voles (Microtus spp.) in apple orchards. Radio transmitters were attached to 38 eastern screech‐owls (Otus asio), 5 barred owls (Strix varia), 3 red‐tailed hawks (Buteo jamaicensis), 2 great horned owls (Bubo virginianus) and 2 long‐eared owls (Asio otus), and the birds' movements were monitored before, during, and after rodenticide applications. Screech‐owls selected woods over alternate habitats for day‐roosting; 73% of daytime locations were in woods. At night, screech‐owls generally were located in woods, orchard, and field‐pasture proportional to their availability, while they avoided cropland. The home ranges of 32 screech‐owls tracked posttreatment included brodifacoum‐treated areas; the proportion of home range treated and habitat use varied among individuals. Minimum mortality was 58% among screech‐owls for which more than 20% of home range was treated, as compared with 17% among those for which less than 10% of home range was treated. Secondary brodifacoum poisoning was the most probable cause of death in six screech‐owls. Of five other screech‐owls found dead posttreatment, four had been consumed by predators and one died of unknown causes. Of six radio‐equipped screech‐owls collected one to two months posttreatment, four contained detectable brodifacoum residue. The fate of 14 of the 32 screech‐owls tracked posttreatment was unknown at the conclusion of radiotracking efforts (63 d after treatment began) because radio contact was lost or the transmitter was dropped; one of these owls was encountered alive in May. Four barred owls tracked posttreatment showed strong selection for woodland habitat and used orchards limitedly; none was found dead posttreatment. One long‐eared owl found dead (not radio‐equipped) was probably killed by secondary brodifacoum poisoning. The results indicate a hazard to screech‐owls and a potential risk to other raptors, given this use pattern and formulation of brodifacoum bait.