WOOD RIVER, Neb. (AP) — A small herd of purebred bison in south-central Nebraska has been growing with the births of at least six calves so far this spring.

The animals have been adjusting to their new home on more than 1,000 acres of tallgrass prairie on Crane Trust land southwest of Grand Island.

The 41 bison — also known as buffalo — were trucked in from Rimrock Ranch near Crawford in the Nebraska Panhandle in January. Trust officials don’t know how many of the 12 mature cows and five heifers will give birth and add to the herd this year.

Experts say that of the 500,000 bison in North America, about 5,000 are genetically pure, descended from bison that have never been bred with cattle.