Baseball-sized hail has been blamed on the death and injury of thousands of birds in the US state of Montana.

More than 11,000 waterfowl and wetlands birds were killed and maimed in the Billing area in the 11 August storm, according to the state's Fish, Wildlife and Parks authority.

The birds were found scattered around Big Lake, located inside a wildlife preserve that serves as a nesting area for dozens of species of ducks.

Huge numbers of birds were said to have suffered "massive blunt-force trauma".

The storm reportedly hurled hail with winds of over 110km/h.

Biologists discovered the dead animals with broken wings, smashed skulls and internal damage.

They said an estimated 20-30% of the birds at the lake were killed or injured.

Five percent of ducks and 30-40% of pelicans and cormorants showed signs of injury.

(Courtesy: Montana Fish, Wildlife, and Parks)