Anticoagulant Residual Concentration and Poisoning in Birds Following a Large-Scale Aerial Application of 25 ppm Brodifacoum Bait for Rat Eradication on Rat Island, Alaska

To eradicate invasive Norway rats, an aerial broadcast of the rodenticide Brodifacoum-25W Conservation was conducted on 2,777-ha Rat Island, within the Aleutian Islands Unit of Alaska Maritime National Wildlife Refuge, between 29 September and 5 October 2008. During subsequent visits to Rat Island (in spring and fall 2009) to check for evidence of remaining rats and to look for evidence of nontarget mortality, personnel found an unexpectedly high number of dead birds: more than 420 carcasses, mostly glaucous-winged gulls and bald eagles, but also included 24 other species. Some carcasses had tissue suitable for analysis and were collected. Seventy bird carcasses were necropsied and cause of death was determined for 50 birds; 45 died of brodifacoum toxicosis. Necropsy determined almost all gulls and eagles had extensive hemorrhaging, consistent with brodifacoum poisoning. All eagles and all but 1 gull tested positive for brodifacoum residues. Positive tests for brodifacoum also were recorded for single specimens of emperor goose, northern fulmar, pelagic cormorant, peregrine falcon, rock sandpiper, and gray-crowned rosy finch. Two Lapland longspurs tested positive for exposure to brodifacoum. Nontarget bird mortality from brodifacoum exposure at Rat Island was higher than predicted in the Environmental Assessment