What Bison heard is a herd of 250–400 American bison in Wind Cave National Park, South Dakota, USA, that  is one of two subspecies/ecotypes of the American bison?
Wind Cave bison herd:  The Wind Cave herd are of the Plains bison subspecies ("Bison bison bison"). Plains bison: The Plains bison ("Bison bison bison") is one of two subspecies/ecotypes of the American bison, the other being the wood bison ("B. b. athabascae").
The Plains bison