Evaluation of the potential hazard to barn owls of talon (brodifacoum bait) used to control rats and house mice

Abstract

During 1980, a radiotelemetry study on barn owls ( Tyto alba ) was conducted to evaluate the secondary hazards of the anticoagulant rodenticide TALON® (containing 0.005% brodifacoum) when it was used for controlling rats ( Rattus spp.) and house mice ( Mus musculus ) on farms. Thirty‐five active nests were located in about 1,100 km 2 (430 mi 2 ) in southwestern New Jersey. Starting on June 9, 1980, radio transmitters with tail clips were attached to 26 adult nesting barn owls and 8 fledged young of the year. In one 20‐km 2 (about 8 mi 2 ) area, which was the most intensively searched, 12 active barn owl nests were located. Of all 35 active nests, 66% were in trees, 11% in barns, 8.5% in silos, 8.5% in chimneys and 6% in miscellaneous locations. Analysis of regurgitated pellets showed that meadow voles ( Microtus pennsylvanicus ) were the most common prey at all sites. Although Norway rats ( Rattus norvegicus ) and house mice occurred in some pellet collections, they made up only a small part of the diet (3.9% rats and 2% mice) of barn owls in the study area. Baiting at farms with TALON was begun on July 24, and ended on September 29, 1980. TALON was not observed to cause or implicated in barn owl mortality during the study. However, trace residues (< 0.05 ppm, the limit of determination) of brodifacoum were found in one electrocuted barn owl. Young barn owls were fledged from at least eight sites where poisoned rodents were available on the farms for at least part of the nesting and feeding period (including pre‐ and postfledging). Furthermore, radiotelemetry confirmed that at least 9, and possibly 12, of the radio‐equipped owls were present on sites posttreatment, when census evaluations showed commensal rodents were present and consuming TALON bait.