During World War II, under the cover of darkness, thousands of homing pigeons were dropped from the sky — unlikely secret weapons in the fight against the Nazis.

It may sound like the product of an over-imaginative mind, but Operation Columba, a clandestine British bid to gain intelligence from occupied areas, was very real.

Between 1941 and 1944, around 16,000 avian agents, hidden in canisters with little parachutes attached, fell to the ground in rural France, Belgium and the Netherlands.

The plucky pigeons sparked hundreds of tiny acts of resistance: villagers sending back messages, tied to the legs of the birds.

One group of villagers, led by a local priest, provided intelligence so valuable it was shown to Winston Churchill — but their brave defiance ultimately led them to a gruesome death.

Gordon Corera, an author and the BBC's security correspondent, has pieced together details of that group, known by the codename Leopold Vindictive.

"I never thought I'd be writing about pigeons," he laughs.

He began his research a few years ago after coming across a news story about the discovery of a dead pigeon's leg in a chimney in Surrey, in South East England.

"I kid you not," he says.

"The dead pigeon's leg had a message attached to it which appeared to come from World War II. I found it bizarre and fascinating."

The message was a series of random letters — a code not even the country's best minds could crack.