Secret Pigeon Service: Operation Columba, Resistance and the Struggle to Liberate Europe

Pigeons were still in use during the Second World War to carry messages from planes and battlefields but Operation Columba set them to work in a more ambitious project gathering intelligence across Nazi-occupied Europe. BBC security correspondent Gordon Corera tells the recently declassified story of the thousands of birds released over Holland, Belgium and France and assesses the value of the information they brought home.