[Note: undated footage]

Extreme weather across central Europe has caused the slaughter of thousands of swallows and swifts trapped before they migrate to Africa and Asia.

Heartbreaking images shared on social media by wildlife workers in Austria show the birds' tiny bodies piled high as whole communities die out from either cold or starvation.

The birds, say experts, have been hit by the double whammy of torrential rain from Storm Boris dropping temperatures and killing off their flying insect food supply.

Too weak to fly, thousands of birds have simply frozen or starved to death weeks ahead of their usual seasonal migration time.

Pictures show dozens of weakened and frozen swallows and swifts huddled on window ledges, crouched on the road, or already dead.
In Austria, many swallows and swifts have been rescued by wildlife volunteers.

One video shows volunteers feeding the desperately weakened birds insects and grubs by hand with tweezers.

But many have been found dead in places like Krems where they were found lying on the anti-roosting spikes used to keep pigeons off buildings.

In normal conditions, swallows would begin their long migration to winter in Africa, while swifts would leave Europe later to fly to Asia.