FORT WORTH, Texas (AP) -- Companies accused of causing migratory bird deaths may be harder to successfully prosecute after a federal appeals court recently ruled that a century-old wildlife protection law only applies if the killing is intentional.

The decision by the 5th U.S. Circuit Court of Appeals runs contrary to two other federal courts' interpretations of the 1918 Migratory Bird Treaty Act and could save companies that operate wind farms, power transmission lines and other methods of energy production millions in research — at the risk of more bird deaths.

"If you have a dead migratory bird, you have a violation. Doesn't matter how, why or when; that's historically been the government's interpretation," said Barry Hartman of K&L Gates, a Washington law firm that represents Duke Energy Corp. "And what the court is saying is that interpretation doesn't extend to unintentional takes."

The U.S. Fish and Wildlife Service has maintained that no intent is required to violate the law, which makes it illegal to "pursue, hunt, take, capture" or "kill" more than 1,000 bird species. But companies have long claimed they should not be held responsible for actions that incidentally cause deaths — up to about a half-million annually by wind turbines alone, the federal agency says.

The nation's appellate courts have starkly different interpretations of the law, resulting in an uneven enforcement across the country, with companies in some energy-rich states facing a narrower liability toward migratory birds than operations in other regions.

In the Sept. 4 ruling, the 5th Circuit said a U.S. District Court was wrong to convict and levy a $2 million fine against Citgo Petroleum Corp. for the deaths of at least 35 protected birds that flew into two large, open-top petroleum tanks in 2003 in Corpus Christi. In overturning the lower court's decision, the 5th Circuit agreed with Citgo that the statute was misinterpreted as covering unintentional bird kills.

The ruling agrees with decisions from the 8th and 9th Circuits that hold the law only applies to the intentional killing of birds, but contrasts with two other circuit rulings — one of which upheld misdemeanor convictions of two Kansas oil rig operators after dead birds were found trapped in oil-treatment vessels.

"It was a very easy charge for the government to bring" without having to prove intent, said Benjamin Cowan, a Houston attorney who represents traditional and renewable energy companies. "They won't be able to do that anymore."

The U.S. Fish and Wildlife Service, which has also used the bird act to regulate wind energy, and the Department of Justice are reviewing the 5th Circuit ruling before deciding whether to appeal to the U.S. Supreme Court, service spokeswoman Laury Parramore said. She added they may also ask the appeals court to reconsider.

The federal service is considering adding new regulations to mitigate bird deaths from energy production.

Since 2013, the Department of Justice has prosecuted two energy companies whose Wyoming wind turbines killed migratory birds, as well as several oil and gas companies in North Dakota whose wastewater pits became death traps for ducks. In 2014, Duke Energy Corp. was sentenced to $1 million in fines and five years' probation and ordered to develop a mitigation strategy for bird deaths at wind farms.

Environmental groups say the ruling by the 5th Circuit — which covers the energy-producing states of Texas, Louisiana and Mississippi — weakens the law's intent in some of the country's most crucial bird habitats; Texas leads the nation in wind energy production. The groups also say the ruling threatens the integrity of treaties signed with Great Britain, Canada, Japan, Mexico and Russia.

The first of these, signed by the U.S. and Canada in 1916, was enacted in an era when many bird species were threatened by the commercial trade of birds and feathers and was intended to protect against the "indiscriminate slaughter" of migratory birds.

Eric Glitzenstein, an attorney who represents the American Bird Conservancy, called the ruling "an extraordinarily narrow" interpretation of the statute.

"Ignoring the greatest threats to migratory bird populations from inherently hazardous industrial activities is really a setback to our commitment to migratory birds in a serious and unfortunate way," he said.