Operation Columba: Secret Pigeon Service

During the First World War, soldiers at the front used pigeons to communicate with those behind the lines, and with tank commanders when their radios failed. In Secret Pigeon Service, his book about the operation, Gordon Corera reports that attempts were also made to send messages ‘written in invisible ink on banknotes and dropped in offertory boxes in Catholic churches, which were allowed to be taken across enemy lines’, and to transmit Morse code with ‘infra-red rays’ by signalling back and forth in front of a hot oven. It was directed by MI14(d), the Special Continental Pigeon Service, a branch of Military Intelligence (MI16), and was the brainchild of Rex Pearson, an agent who spent the interwar years at Unilever in Switzerland as cover for his work for the Z organisation, a parallel intelligence network run by MI6.

Source: www.lrb.co.uk