The deaths of 200 starlings in Yankton, South Dakota this week is no mystery — they died as the result of poison set out by the U.S. Department of Agriculture, an official said on Thursday.

USDA wildlife biologist Ricky Woods explained that a large group of starlings was causing problems in a north Nebraska cattle feedlot, eating the feed and leaving waste on both the feed and equipment. So the USDA put out DRC 1339 poison for the birds, Woods said.

“Lethal means are always a last resort,” said Woods. “In this situation it’s what we had to do.”

Woods said most of the birds died near the site of the feed lot, but about 200 were strong enough to fly about 10 miles north to Yankton, where they died, puzzling some local residents. He could not say how many birds died altogether.

Woods said putting out poison for birds is not common, but sometimes is necessary. “It depends on the situation,” he said. He said the poison would not harm another animal which ate one of the birds, such as a cat or a hawk.

Large-scale bird deaths have recently been reported in the south, including 5,000 red-winged blackbirds which fell from the sky in Beebe, Ark. Wildlife experts determined that those birds died as a result of being startled by New Year’s Eve fireworks, and then flying into buildings and trees.