BISMARCK, N.D. (AP) — Six oil companies fighting federal charges that their oil pits killed 28 ducks and other birds intend to request separate trials, and at least one may argue that investigators gathered some evidence illegally.

The companies, which were arraigned Thursday in U.S. District Court in Bismarck, face fines of up to $15,000. U.S. Magistrate Charles Miller Jr. set an Oct. 25 deadline for the companies to file pretrial arguments.

Miller said it was unclear whether the companies had a right to a jury trial on the misdemeanor charge, and Gary Delorme, an assistant U.S. attorney, said he would oppose the request.

"This may technically not be a petty offense," Miller said. If it is not, the companies may seek a jury trial, he said.

John Russell, an attorney for one of the companies, Continental Resources Inc., said he may request a hearing to argue that some evidence gathered against the company should be excluded. Russell did not elaborate and did not comment afterward.

The federal law that is being used to prosecute the companies was originally approved to prohibit illegal hunting, but its reach is considerably broader.

The Migratory Bird Treaty Act has been used to prosecute a barge operator who caused a large oil spill that resulted in bird deaths, and a farmer who accidentally poisoned some ducks when he sprayed pesticide on an alfalfa field.

The law does not require prosecutors to show the companies were negligent or intended to do anything wrong to win a conviction.

The companies in the dock are Continental, of Enid, Okla.; Petro-Hunt LLC, of Dallas; Brigham Oil and Gas LP, of Austin, Texas; Newfield Exploration Co., of The Woodlands, Texas; ConocoPhillips, of Houston; and Fidelity Exploration & Production Co., of Denver. Fidelity is a unit of MDU Resources Group Inc., which is based in Bismarck.

The arraignment of a seventh company, Slawson Exploration Co. Inc., of Wichita, Kan., was delayed until Oct. 21. It faces an identical charge for allegedly killing 12 birds from May 6 through June 20.

All seven companies have previously been fined for violating the same law, records say. Fidelity was assessed the largest fine, a $44,025 penalty after 41 ducks and three shorebirds were found dead in gas production waste pits near Green River, Utah.

While an oil well is being drilled, North Dakota rules allow workers to dump a nasty soup of diesel fuel, slippery chemicals and ash into a nearby waste disposal pond, which ducks can sometimes mistake for a good place to land.

North Dakota laws now allow the dumping of liquid waste into open pits, which can be 15 feet deep and as large as a swimming pool, while an oil well is being drilled.

If the pit is left open more than three months after the well drilling is finished, it must be fenced off and have a net stretched across its surface, state law says. The pits must be filled in and the land reclaimed within a year.

Regulators have drafted proposed rules that would ban the dumping of liquid waste into open pits. Rock cuttings and other solid waste could still be discarded into a pit.

Richard Grosz, a U.S. Fish and Wildlife Service agent, said in a court filing that 500,000 to 1 million birds are killed annually across the United States as the result of oilfield production.

In a sworn statement, Grosz said hawks, owls, ducks and other birds mistake the waste pits for water ponds. They become fouled with oil, and their feathers lose their ability to shed water, often causing the birds to become waterlogged and drown.

Birds that attempt to drink from the pits can fall in, and their steep, slippery slopes "make it almost impossible for trapped wildlife to escape," he wrote.

"The sticky nature of oil entraps birds in the reserve pits and they die from exposure and exhaustion," Grosz's statement says. "Birds that do manage to escape die from starvation, exposure or the toxic effects of oil ingested during preening."