It kept its secret for decades. It perished in the process. It died, experts say, a valiant death, most likely on a hush-hush mission over wartime France, and was then, like so many others, forgotten.

But now, decades after the final flight of military carrier pigeon 40TW194, the bird’s secret message has become a matter of state and the grist of headlines. After a concerted campaign by pigeon fanciers, the encrypted message, which had been folded into a scarlet capsule on the pigeon’s leg, has now been sent to Britain’s top-secret GCHQ listening post and decoding department outside Gloucester to the west of London.

There, 40TW194’s World War II secret might finally be revealed. Or maybe not. “We cannot comment until the code is broken,” said a spokesman for GCHQ, which stands for Government Communications Headquarters. “And then we can determine whether it’s secret or not.”...