Baby makes 30, more or less. The small herd of American plains bison at Minneopa State Park in Mankato has swelled this spring when a record 10 babies were born, joining the herd of about 20.

"Number 10 was born yesterday," park manager Craig Beckman said on Tuesday, adding that another calf could be on the way. Beckman has been eyeing the herd for the surest sign that a calf is coming: visible udders on cows (female bison are called cows; males are bulls).

"Unlike cattle cows, you can almost never see an udder on a female bison until they are coming close to having a calf. This morning, I saw a couple of cows that didn't have calves. One had an obvious udder showing," he said. Bison births generally occur between early May and mid-June.

The babies enter the world at about 40 to 50 pounds, wobbling on unsteady legs to drink milk. But "they'll be in the 400-pound range within six months," Beckman said. The caramel-colored cuties grow quickly, eventually turning stout, shaggy and dark brown like the adults in the herd.

At Minneopa, just west of Mankato off Hwy. 169, visitors can drive through the 331-acre bison range for a chance to see the animals up close. Bison Drive Road is open Thursday-Tuesday from 9 a.m.-8:30 p.m.

Beckman offered these viewing tips: Go in the morning or evening, when the animals are generally most active. Know that a warm day can be great since the animals cool off at a water source near the road. Hike the path that rings the bison-range fence.

He also had a few important warnings: Keep pets on leashes. Keep quiet and move gently since bison can be skittish, and give the animals a clearance of at least 75 feet. Finally, never forget that bison are unpredictable and surprisingly fast and mothers are protective of their young, so when you are in a vehicle driving through the bison range, stay inside.

Send your questions or tips to Travel Editor Kerri Westenberg at travel@startribune.com, and follow her on Twitter: @kerriwestenberg.