During 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.

In a 7—year study on Black Mesa, Colorado, elevation 10,000 ft 2,4—D herbicide treatment initially reduced forb abundance and northern pocket gopher (Thomomys talpoides) populations 80% to 90%. Both forbs and pocket gophers remained at low levels on one area but returned to pretreatment levels on the other area. The decline in pocket gopher numbers was caused by an inability to survive where their preferred food (forbs) was eliminated by herbicide treatment. The decline was not caused by movements of the animals from sprayed areas, or by direct or indirect toxicity of 2,4—D. The common grasses offer, at best, only a marginal diet for northern pocket gophers; only succulent grasses or those bearing corms or rhizomes yield a subsistence diet. Limited sampling did not indicate that the spray treatment affected the numbers of other small mammals.

During 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.

Relatively few field studies have been conducted to evaluate hazards to wildlife from rodenticide use. In the USA, field studies have been conducted on both acute and chronic compounds, including zinc phosphide, strychnine, 1080 (sodium monofluoroacetate), and the anticoagulants diphacinone and brodifacoum. Techniques employed in these studies have included carcass counts, direct counts, indirect counts, nest site monitoring, radiotelemetry, habitat and diet evaluation, necropsy, and residue analysis. Although zinc phosphide generally is not secondarily hazardous, it can pose primary hazards, especially to seed‐eating and gallinaceous birds and waterfowl. Strychnine can pose primary hazards, such as to seed‐eating birds; secondary poisoning may be minimal unless predators consume stomach or cheek‐pouch contents of poisoned prey. 1080 can result in primary poisoning, but it especially can pose a secondary hazard to mammalian predators; the risk to raptors is minimal. Anticoagulants are toxic both primarily and secondarily; they can pose a substantial hazard to raptors. Gallinaceous birds, however, are quite resistant to them. Hazards associated with any one rodenticide may vary significantly depending upon use pattern (e.g. commensal vs field). Environmental concerns over rodenticide hazards to wildlife are increasing greatly and will affect future registrations.

No abstract