SECRET Pigeon Service author Gordon Corera tells Laura Smith the Honest Truth about Operation Columba during the Second World War, which saw thousands of pigeon “spies” dropped behind enemy lines so resistance fighters could send back valuable information on German defences.

Why did you decide to write this book?

I stumbled across Operation Columba by chance when I was covering a quirky news story about a dead pigeon’s leg found in a chimney in Surrey.

Attached to the leg was a message which had stumped GCHQ’s top code-breakers.

No one was sure who the pigeon had been sent by, and everyone seemed surprised to find pigeons had been used in the Second World War.

In the National Archives I eventually found a file containing tiny pink slips of paper – messages from ordinary people living under Nazi rule in Occupied Europe that had been brought back by pigeon.

Why did British Intelligence use homing pigeons during the war?

Radios were rare at the start of hostilities and getting them into enemy territory was hard. If you were found with one you were in real trouble.

Homing pigeons offered a way of reaching ordinary people.

Sometimes the messages they sent back reached Britain in a few hours. Messages sent by human couriers often took months.

How did Operation Columba work?

These were homing pigeons supplied by British civilian pigeon fanciers. They were dropped in containers by the RAF over Occupied Europe with a questionnaire asking for intelligence on Nazi movements.

Answers would be written on a piece of paper, pushed into a small container clipped to the bird’s leg.

Then the bird could be released and use its homing instinct to get back to its home loft.

The volunteers would alert the War Office when a message arrived and it would be sent into a special team in British intelligence for analysis.

Everyone has heard of MI5 and MI6 – they were MI14 (d).

What was the scale of the operation?

About 16,000 pigeons were dropped from 1941 to 1944.

More than a thousand messages were brought back.

What dangers did the birds face?

Hungry locals would simply eat them or they would be handed in for a reward to the Gestapo.

The Germans sometimes used snipers to shoot them but the biggest menace were the hawks Germans trained to kill them.

What about the informants?

There were brave people who took use risks in sending material by pigeon. In some cases they paid for it with their lives.

The group I focus on, who called themselves Leopold Vindictive, were ordinary people doing extraordinary things and they collected amazing intelligence.

How crucial were pigeons to the war effort?

Pigeons did not win the war but they did bring back valuable information.

Some of it helped identify German radar stations used to shoot down RAF planes which were completely unknown before the pigeons provided locations.

In other cases they provided details of V1 rocket launch sites targeting London or put British intelligence in touch with underground resistance networks.

Some pigeons made incredible journeys. One of my favourites was Winkie, who was released without a message from an RAF plane that crashed in the sea.

But someone was able to work out from her condition and route where the plane had crashed and the crew was rescued.

Did they receive any recognition?

Some pigeons won the Dickin Medal for carrying out remarkable flights, including as part of Operation Columba.

This was sometimes when they flew particularly long or hazardous journeys or brought back vital intelligence.

One was a pigeon owned by Mary Manningham-Buller which brought back information from France.

Mary’s daughter, Eliza, went on to be a recent head of MI5.

Secret Pigeon Service, published by Harper Collins, is out now.