Brodifacoum poisoning of avian scavengers during rat control on a seabird colony

Langara Island, at the north-western tip of British Columbia's Queen Charlo tte archipelago, was once nesting grounds for an estimated 500,000 seabirds . However, infestations of Norway rats (Rattus norvegicus) and their predat ion of eggs and breeding adults have caused extirpation or serious declines of all seabird species. By 1993, the breeding population of ancient murrel ets (Synthliboramphus antiquus) had declined to 10% of its historical size. Successful eradication of rats on smaller New Zealand islands using the an ticoagulant brodifacoum prompted its application on Langara Island. The isl and is also home to breeding bald eagles (Haliaeetus leucocephalus), peregr ine falcons (Falco peregrinus), and other wildlife. No comprehensive studie s of non-target impacts and potential for secondary poisoning were done dur ing similar operations elsewhere; thus, in 1994 and 1995 we initiated a two -year study into the risk of secondary poisoning to non-target species. Dur ing 1994, rat carcasses were laid out with motion sensor cameras to identif y potential scavengers. Ravens, northwestern crows and bald eagles were pho tographed at carcasses, and therefore at risk of feeding on rats that die a bove ground. During the baiting program, 19 rats were radio-tagged to deter mine the proportion dying above ground, and thus available to predators/sca vengers. Ravens were found poisoned both from feeding directly on the bait, and predating/scavenging poisoned rats. Bald eagles were trapped and blood sampled for brodifacoum residue analysis and prothrombin time evaluation; 15% of the sampled population showed detectable residues but no adversely-a ffected birds were found. We conclude that the use of brodifacoum for rat r emoval on seabird islands poses a clear risk of secondary poisoning to avia n scavengers. This risk must be weighed against the benefit of rat removal programs.