Last month we reported on bald eagles and other birds found dead after a rat eradication project in Alaska’s Aleutian Islands. The National Wildlife Health Center in Madison, Wis., has confirmed that the birds were casualties of brodifacoum, the poison used in bait scattered around Rat Island by helicopter.

“Every one of the liver samples tested positive for brodifacoum,” U.S. Fish and Wildlife Service spokesman Bruce Woods told Scientific American. Fish and Wildlife law enforcement agents are investigating whether there were any egregious errors and to assess that the poison drop was conducted according to an approved protocol, Woods said.

For two centuries, invasive rats on the island have ravaged populations of ground-nesting seabirds. In September, Island Conservation, the Nature Conservancy, and the U.S. Fish and Wildlife Service dropped rat poison from helicopters after an environmental assessment concluded birds were unlikely to be harmed because the rodents would perish in their burrows.

A recent census found puffins and other seabirds were returning to nest on the island in the absence of rats. But wildlife workers also discovered corpses from 43 bald eagles, 213 glaucous-winged gulls, and several other species. The scientists believe gulls may have consumed the poison cakes and were then preyed upon by eagles.

The wildlife lab reports that two bald eagles, two glaucous-winged gulls, one peregrine falcon, and one rock sandpiper all tested positive for the poison. The team plans to analyze more tissue and soil samples. Another team will return to the island in early August to look for any further mortality.

Conservationists still hope to eradicate rats from other islands in the Aleutian chain, but they may have to modify their approach. “When we get all the information,” Woods says, “we will attempt to figure out what we can do better.”  

Image of bald eagle courtesy Alaskan Dude via Flickr