U.S. Army
The U.S. Army used specially trained homing pigeons to carry messages during WWI and WWII. They were considered an undetectable method of communication. Fort Monmouth, New Jersey was the home of the U.S. Army Pigeon Breeding and Training Center from 1917 until 1957. A small capsule would be placed around the leg of the pigeon and a paper message was put into the capsule. The bird would be released to fly to its home loft with the message.
Pigeons have been known to fly hundreds of miles in a day with a mile a minute being the average speed. Military historians claim that over 90% of all messages sent by the US Army using pigeons were received. Pigeons were also used to carry maps, photographs and cameras. The birds are credited with saving thousands of lives. The U.S. Army discontinued using pigeons as message carriers in 1957 due to more modern and faster transmission methods.
British Army
During World War I the British Army had a unit called the Carrier Pigeon Service (CPS) which was led by Lt. Col. A.H. Osman. Carrier pigeons were used by the Brits during the Second Battle of Ypres in May of 1915. The Carrier-Pigeon Service was only used when telegraph and telephone communications failed and was soon overtaken by the development of Wireless Telegraphy (i.e. Radio), further limiting their usage; hence, they were only used for emergency or espionage purposes. The avian unit saw further success at the Battle of the Somme and at Verdun, often against screens of poisonous gas and heavy shelling from the opposition.
During the D-Day invasion of World War II, many soldiers were sent with a pigeon beneath their coats. This was a period of radio silence, so the use of pigeons for relaying messages was optimal. The pigeons were able to send back information on German gun positions on the Normandy beaches. Thirty-two pigeons were awarded the prestigious Dickin medal, Britiain's highest award for animal valor. Recipients included a bird named "G.I. Joe," who flew 20 miles in 20 minutes with a message that stopped U.S. planes from bombing an Italian town that was occupied by British forces.
Australian Army
The Australian Army made extensive use of pigeons in WWII in New Guinea and the islands. Surrounded by rugged terrain and unusual atmospherics, wireless communications often failed. Bert Cornish was the Aussie behind their pigeon forces. Some 13,500 birds were used in the Pacific Theater by the Australian Army. They awarded two Dickin medals to pigeons that aided them in the war.
Sources:
Lawless, Jill. The Associated Press. "Auction Focuses on Pigeon Warriors: Memories of elite WWII animal force included in offerings." January 6, 2008.
Osman, Lt. Col. A.H., Pigeons in the Great War: A Complete History of the Carrier Pigeon Service during the Great War, 1914 to 1918 (London, 1928)
Williamson, Mitch. "Homing (Carrier) Pigeons in War." October 11, 2007.