For bison, the birthing season is often called the "calving season." Research has documented that the bison calving season often occurs between mid-April to early-July, across multiple herds in different locations, and with many births concentrated within only several weeks.
Bison females (cows) begin reproducing around 2-3 years of age, giving birth to a single calf each spring after a 9.5-month gestation period. Calves are born with their eyes open. They weigh from 33 to 55 pounds (15 to 25 kilograms). Calves walk and graze within a week of birth and follow their mothers and the herd over long distances.
According to the U.S. National Park Service, 80% of bison have a birthday in April or May. This year, I visited Custer State Park and adjacent Wind Cave National Park from May 19 through 20. Indeed, it appeared that most calves within the parks were born in April.

At Fisherman’s Flat within Custer’s Wildlife Loop Road, I observed a herd of about 200 animals. Several dozen calves within the herd appeared to be about the same size. This indicates that they were born at roughly the same time.[ Ссылка ]

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