Fort Peck, Montana — At the Fort Peck Indian Reservation in Montana, a bison calf is the newest member of one of the first herds to roam the Assiniboine and Sioux lands in more than a century.

"My generation never got to grow up around buffalo," Robbie Magnan, who manages the reservation's Game and Fish Department, told CBS News. "Now, my children and my grandchildren are able to witness them being on our homeland." 

Magnan's department oversees a bison herd that started more than 20 years ago and has now grown to about 800.

According to the U.S. Fish and Wildlife Service, tens of millions of bison once roamed North America, but their populations were reduced to the brink of extinction in the 19th century during the United States' westward expansion, leaving only a few hundred left.

The Fort Peck Buffalo Program is part of a project to reintroduce bison to tribal lands throughout the U.S. using animals from Yellowstone National Park.

Due to brucellosis, a bacterial disease that can infect and lead to stillbirths in cattle, bison are not protected outside the park, meaning they can be slaughtered once they leave. As a result, the only way bison are able to safely leave Yellowstone is by completing an up to three-year quarantine that culminates at a testing facility in Fort Peck.

Magnan and his team showed CBS News how it corralled 76 bison through what it calls "running alleys" to undergo testing.  

The quarantine program has protected hundreds of animals from slaughter and reintroduced bison to 24 tribes across 12 states. But advocates say it is unnecessary since cattle have never contracted brucellosis from wild bison.

"I feel sad whenever animals in the corral system, and buffalo stress out very easily," Magnan said. "But in order to save your life, I gotta do this. And then I don't feel so bad. I know what I'm doing is gonna be for the greater good."

The U.S. now has about 420,000 bison in commercial herds, according to USFWS, and another 20,500 in conservation herds. 

In:
• Bison
• Montana
• Yellowstone National Park