A long-term study nailed down January as the month when Yellowstone grizzly cubs are born. Grizzly cubs in Gates of the Arctic were typically born in February, roughly a month after the Yellowstone cubs, she said. Yellowstone grizzly bear sow with cub in spring. That could be why, for example, January is the birth month for Yellowstone grizzlies. Grizzly bear cubs in Yellowstone on May 8, 2021 (KAR photography via Alamy)‘These Populations Are Still Growing’Life is tough for grizzly cubs.