BOZEMAN, Mont. (AP) — An investigation has started in southern Montana after seven trumpeter swans were found dead at a pond near Bozeman, wildlife officials said.

The state Department of Fish, Wildlife and Parks received a report of dead swans between the towns of Belgrade and Manhattan on Friday, the Bozeman Daily Chronicle reported Monday.

Biologists and wardens collected the animals’ bodies Saturday, department spokesman Morgan Jacobson said.

“We still don’t have a cause of death,” Jacobson said, adding that the agency has not ruled out any causes including poaching or the swans eating moldy grain.

Trumpeter swans are on the state’s species of concern list, according to Montana Field Guides’ website. The list helps resource managers make decisions about species conservation and data collection priorities.

Trumpeter swans are the largest waterfowl in North America, experts said. The birds can be up to 5 feet in length, have a wingspan up to about 7 feet and weigh over 20 pounds.