One of seven oil companies charged with killing migratory birds during drilling operations in North Dakota has agreed to plead guilty and pay $12,000.

Slawson Exploration Co. Inc., of Wichita, Kan., was charged under the Migratory Bird Treaty Act with for killing 12 birds that died after allegedly landing in oil waste pits in western North Dakota from May 6 through June 20. Under a plea agreement filed in federal court Monday, Slawson will pay $12,000 — or $1,000 per bird — to the nonprofit National Fish and Wildlife Foundation.

The maximum penalty for each misdemeanor charge under the Migratory Bird Treaty Act is six months in prison and a $15,000 fine.

U.S. Attorney Timothy Purdon said he could not comment on plea deal because it still must be approved by U.S. District Judge Daniel Hovland in Bismarck. Stu Kowalski, an attorney for Slawson, also declined to comment.

Six other oil companies have pleaded not guilty to charges that their oil waste pits killed birds. They were arraigned last month in U.S. District Court in Bismarck and have until Tuesday to file pretrial arguments.

Slawson accounted for the bulk of the 28 dead birds discovered by federal wildlife officials in uncovered waste pits operated by the companies in May and June. Most of the birds were ducks and none were identified as being endangered. Killing endangered species can lead to felony charges that bring fines of up to $250,000.

Companies in North Dakota are required to cover the so-called reserve pits with netting if they are open for more than 90 days after drilling operations. The waste pits, which can contain oil, diesel, drilling muds and chemicals, are about the size of a large swimming pool, and birds sometimes mistake them for a good place to land.

Also charged in the case are ConocoPhillips Co., of Houston; Newfield Production Co., of Houston; Brigham Oil and Gas LP, of Williston; Continental Resources Inc., of Enid, Okla.; Petro Hunt LLC, of Dallas; and Fidelity Exploration & Production Co., of Denver;

Records show all seven companies have previously been fined for violating the Migratory Bird Act. Fidelity, a unit of Bismarck-based MDU Resources Group Inc., had the biggest sanction, a $44,025 penalty after 44 birds were found dead in waste pits near Green River, Utah.

North Dakota regulators are considering banning the oil waste pits and requiring companies to recycle liquid drilling waste amid a spate of toxic discharges and an increasing number of migrating birds that have died by mistaking the polluted ponds for fresh water. State officials have said companies have sometimes foregone netting because fines can be cheaper than installing and maintaining netting.

North Dakota, the nation's fourth-largest oil state, produces about 450,000 barrels of oil daily from about 6,000 wells.

State regulators in June levied $3 million in fines against 20 companies that failed to protect oilfield waste pits from spring flooding. About 10 percent of the state's 500 waste ponds were swamped by meltwater after one of the state's snowiest winters on record. The waste pit breaches came despite regulators' warnings that they could happen.

State health officials have said other fines are pending in the swamped waste pit incidents.