DES MOINES, Iowa (AP) — A government wildlife researcher says he’s found that rabbits and skunks can become infected with the bird flu virus and shed it enough to infect ducks, evidence that small mammals could contribute to the spread of bird flu on farms.

National Wildlife Research Center biologist Jeff Root says his experiments have shown striped skunks and cottontail rabbits in captivity in his laboratory have indirectly transmitted a strain of bird flu to mallard ducks after they’ve shared food and water sources. Root says it’s important now to figure out how likely it is such transmission across species happens in the wild.

Last year bird flu resulted in the death of 48 million birds in 15 states. Scientists hope to find how it gets from wild birds to farms and spreads.