Prosecutors hope to use a rarely enforced federal law to punish two Iowa pilots whose low flying disturbed thousands of resting migratory birds in a case that centers on this question: Is it a crime to harass animals?

In a case drawing attention from bird lovers, two Des Moines men have been charged with violating a federal law that prohibits using aircraft to harass animals. A judge is expected to decide soon whether the Airborne Hunting Act is constitutional. Attorneys for the two men, Paul Austin and Craig Martin, say it's not.

Among the questions being debated: Are birds capable of feeling harassment? And if harassing birds is a crime, wouldn't Capt. Chesley "Sully" Sullenberger have violated the law when he accidentally struck a flock of geese before famously landing his plane safely on the Hudson River?

Both sides agree Austin and Martin were flying low on Nov. 16 as they passed over Saylorville Lake, a reservoir north of Des Moines known for birdwatching. Tens of thousands of pelicans, ducks, geese and other birds stop there every fall to rest and feed before continuing south.

A natural resources specialist for the U.S. Army Corps of Engineers, which manages the lake, saw the two planes pass about 20 feet above the water, disrupting thousands of white pelicans and other birds. Once the birds settled on another part of the lake, the planes passed by again, sending them back into flight, prosecutors said.

Natural resources specialist Jonathan Wuebker snapped photographs and eventually cited Austin and Martin for flying "in a careless, negligent or reckless manner" over protected land.

Then in February, a grand jury indicted the men on charges of violating the Airborne Hunting Act, which carries up to one year in jail. Prosecutors also aim to seize their small planes _ a 1974 Magnus Bowers Fly Baby and a 1946 Aeronca.

Prosecutors say the law applies even though the pilots weren't hunting because its ban on harassment makes it a crime "to disturb, worry, molest, rally, concentrate, harry, chase, drive, herd, or torment" animals with a plane. Wuebker compared it to using a car to chase deer through a field.

"When it is intentional or blatantly obvious, I would definitely consider that harassment. But that's not my decision," he said, noting trial is scheduled for May 30.

Austin and Martin have asked a judge to dismiss the case, arguing the law is unconstitutionally vague. In a court filing, defense attorneys said it "seems doubtful" that animals experience the kind of human emotional response necessary to feel harassed. And how can pilots know?

"Flying is what birds do. Who can say if the bird is pleased or annoyed to have taken flight? Indeed, who can say whether the bird's flight was the result of any cognition and not just impulse?" they asked.

Austin's attorney, William Ortman, said Monday that the law doesn't draw a clear line between legal and illegal behavior.

In court documents, defense attorneys noted that planes routinely strike birds on accident. They cited the 2009 incident in which Sullenberger successfully ditched US Airways Flight 1549 in the Hudson River after a flock of geese hit it following takeoff. Under the government's theory, they argued, Sullenberger "likely `harassed' the flock of birds that downed his plane, and he probably `harassed' fish when he arrived in the Hudson."

Martin's attorney and a spokesman for the prosecution did not immediately return phone messages.

In a filing last week, assistant U.S. Attorney Cliff Wendel rejected the idea that Congress meant to punish pilots for something "so common and unavoidable" as bird strikes. But he said reasonable pilots know that flying planes at a low altitude above thousands of resting birds would be considered harassment. It's like one famous legal definition of pornography, he said: you know it when you see it.

People know when they're harassing someone, "whether it's an older brother picking on his little sister; a baseball pitcher intending to hit the batter; or two pilots, flying their airplanes 20 feet above the ground, while making two passes that both times cause 6,000 migratory birds to flee from their resting place," Wendel wrote. Robert Johns, a spokesman for the American Bird Conservancy, said Saylorville Lake was an important resting spot for migrating white pelicans and "this sort of behavior should not be tolerated."

"The pilots in this case showed a callous disregard for the birds, the natural environment, and anyone who might have been peacefully enjoying them," he said.