Britain had discovered that German spies were using pigeons to send messages to bases over the English Channel during World War II.
S.S. leader Heinrich Himmler was a pigeon fancier, and in fact was the president of the German National Pigeon Society.
The Nazis confiscated pigeons from their owners to use for the war effort, and baskets of pigeons were smuggled into England by human smugglers, submarines, and parachutists.
To counter the menace, the MI5 spy agency tamed and trained its own crack force of peregrine falcons.
They patrolled the air over the British coasts in two-hour shifts, and took down any pigeons flying off toward the mainland.