Because Theodore Roosevelt National Park is fenced, officials have to prevent the bison herds from getting too large to ensure there’s enough available forage for the grazing animals, said Blake McCann, wildlife biologist. The target is to keep the herd at about 300 to 500 at the South Unit and less than 300 at the North Unit, he said.

The park held a roundup at the North Unit last year and expects to take next year off, said Eileen Andes, chief of interpretation and public affairs.

This week, park officials are using two helicopters to guide bison toward a corral system at the edge of the park.

“They actually bring them fairly slowly toward the facility so the animals are walking fast, but we don’t want to get them too excited and too agitated,” Andes said.

The bison remain in a fenced-off pasture until they are guided through a series of gates and pens. Each animal is led into a hydraulic squeeze chute, where a veterinarian takes blood samples to verify the bison are disease-free.

Park Service staff also collect hair samples from the tail for genetic testing that is part of an ongoing research project. McCann also determines the age of the animals by looking at their teeth. The yearling and 2-year-olds are candidates for being removed from the herd, McCann said.