BILLINGS, Mont. - One of the last and largest wild bison populations in North America has begun its migration out of Yellowstone National Park into southwestern Montana, where they are being hunted and face government-sponsored slaughter as part of a population reduction program.

Hundreds of the animals have moved in recent days into the Gardiner basin along the park’s northern border, Yellowstone spokeswoman Morgan Warthin said. The animals leave the park in winter to graze at lower elevations.

Capturing the bison for possible shipment to slaughter “could happen at any point given the large number of bison in the basin,” Warthin said.

Because this year’s migration occurred somewhat late, the park has only a narrow window to capture animals before the weather warms and they return to higher elevations inside the park, Montana Fish Wildlife and Parks regional supervisor Mark Deleray said Wednesday.

State and federal officials want to reduce Yellowstone’s bison herds this winter by up to 900 animals under an agreement intended to shield Montana’s cattle industry from the disease brucellosis. Yellowstone had just over 4,800 bison as of last summer.

The population reduction would come through a combination of hunting, slaughter and placing up to 110 animals into quarantine for potential relocation at a later date.

Native American tribes from Oregon, Idaho, Montana and Washington state with treaty rights in the Yellowstone region are holding hunts that have killed more than 50 bison this year, Deleray said.

Montana’s state-sponsored bison hunting season ended on Feb. 15 with four animals killed.

The slaughter of park bison has long drawn objections from wildlife advocates, members of Congress and some Native Americans. It occurs under a 2000 agreement between the state and federal officials that’s meant to reduce the chances of brucellosis infecting cattle.

The disease causes animals to prematurely abort their young and has been eradicated in the U.S. outside the Yellowstone region.

There have been numerous cases of cattle in the Yellowstone region being infected with brucellosis by diseased elk, but no recorded transmissions from bison.