A severe cold spell has caused a "loon fallout" across the American Midwest.

Migrating loons, a type of aquatic bird with relatively small wings, have dropped out of the sky and become stranded in fields and ponds.

Unseasonably cold weather for late-April across the Midwest caused ice to form on the birds' wings as they migrated at high altitudes from the Gulf of Mexico to Canada.

They can fly up to altitudes of 1200 - 1800 metres, where temperatures are often close to freezing. During this cold spell however, temperatures are over 10 degrees below average.

The fallout was reported across at least six cities in Wisconsin, and in huge numbers which raised alarm bells.

They have been stranded on land as their small wings and short legs mean they cannot walk far. A rescue operation is currently underway to save the loons.