Carrier Pigeons
The British Army Pigeon Service was widely used for routine communications during the First World War. Trained pigeons were particularly useful once troops had advanced or retreated beyond prepared field telephone lines. They were also used as as back-up to radio on warships and seaplanes.
Army pigeons were willing to fly through heavy bombardment. However, pigeons are trained to fly from where they are released to a known base. Therefore they could only be used from the frontline to the rear, not the other way round.
Primary Sources
(1) General Henri-Philippe Petain received a carrier pigeon message from French forces defending Fort Vaux at Verdun (4th June, 1916)
Still holding out, but we are sustaining a very dangerous gas and smoke attack. Must be relieved soon. Send us visual communication through Fort Souville, which does not answer our appeals. This is our last pigeon.