The bison live on the property of the Fermi National Accelerator Laboratory. Spring is the birthing season for the animals, which can be seen frolicking in the waters left behind after last week's storm on a youtube video.
They have a 9-month gestation period, and babies can weigh up to 66 pounds.
The lab has 24 bison, and will sell any females that are born to keep the number in the 20s. A part-time worker manages the herd.
The babies can grow up to 6.5 feet tall, and weigh as much as 2500 pounds.