Federal wildlife inspectors have found hundreds of dead birds in a piece of equipment used by the oil drilling industry, and authorities said they hadn't realized until now that it was a danger to migratory birds.

The birds were found in cylindrical tanks — typically 20 feet tall and 5 feet around, topped with a half-foot-wide chimney — called heater treaters.

The tanks are routinely used in oil fields to separate gas, oil and saltwater pumped from the ground.

"I don't think anybody really realized how big a problem this could be," said John Brooks of the U.S. Fish and Wildlife Service on Wednesday.

Federal wildlife officials in Kansas are in the early stages of getting the information disseminated nationwide, Brooks said.

The problem came to light after agency received a complaint from a resident.

After an initial investigation, Brooks assembled federal wildlife inspectors and agents from the Kansas Department of Parks and Wildlife to search heater treaters in state oil fields for violations of the Migratory Bird Treaty Act.

In two days, they inspected 150 heater treaters, finding the remains of between 350 and 400 birds in about half of them, he said. The birds included meadowlarks, flickers, starlings and blackbirds.

Wildlife officials have given Kansas oil producers until of the end of this year to fix the problem, said Jon Callen, president of the Kansas Independent Oil and Gas Association.

He estimated it would cost between $100 and $150 to retrofit each heater treater. But because thousands of state oil producers use the devices, the industry's cost will likely be more than $1 million and could reach as high as $10 million in Kansas alone, he said.