Inasmuch as historians may like to refer to World War II as "the first hi-tech war", it must be remembered that some of the most ancient tools were still put to use with the greatest efficiency. The attached article gives a very brief outline concerning the W.W. II use of carrier pigeons and the goings on at Camp Crowder, Missouri, where these birds were trained. Throughout the entire course of the war, 50,000 carrier pigeons were trained by the U.S. Army.
"Since 1400 B.C. these birds have acted as couriers; they are the oldest instruments of war still in use. Although they form only a small part of our tremendous Signal Corps resources, the Army maintains a corps of expert pigeoneers who have rendered their birds, by scientific training and breeding, ten percent stronger than those used in World War One."
You might also enjoy reading this article about the carrier pigeons of W.W. I.