DENVER -- Dozens of slinky, ferocious and rare ferrets are settling in and making babies at a wildlife refuge outside Denver one year after they were released there.

The U.S. Fish and Wildlife Service counted 47 black-footed ferrets last month at the Rocky Mountain Arsenal National Wildlife Refuge. That includes 71 per cent of the original 28 captive-born ferrets released there in 2015.

Kimberly Fraser of the Fish and Wildlife Service black-footed ferret program calls that a phenomenal survival rate.

Searchers also found nearly two dozen black-footed ferrets that were born at the refuge, a promising sign for the campaign to bring the animals back.

Black-footed ferrets were once thought to be extinct but a small colony was found in Wyoming in 1981. They're classified as an endangered species.