BILLINGS, Mont. — Yellowstone National Park administrators say the park's bison population is up to an estimated 4,230 animals, including 600 calves born this year.

The park's two herds increased in size by a combined 14 percent over last year's count.

The peak bison population was seen in 2005, when about 5,000 were recorded.

During harsh winters, bison attempting to migrate outside the park have often been captured and killed to prevent the possibility of them spreading disease to cattle.

A proposal by the Montana Department of Fish, Wildlife and Parks would ease restrictions and allow them to roam free year-round.