Avian carriers achieved a message delivery rate of 95% in the first World War, and were reported to reach 99% success in an Army study published in 1944.
In an article at At War on the Rocks, Dr. Frank Blazich provides a brief overview of the military use of homing pigeons and argues for their reintroduction as a response to electronic warfare.
Considering the storage capacity of microSD memory cards, a pigeon’s organic characteristics provide front line forces a relatively clandestine mean to transport gigabytes of video, voice, or still imagery and documentation over considerable distance with zero electromagnetic emissions or obvious detectability to radar. These decidedly low-technology options prove difficult to detect and track. Pigeons cannot talk under interrogation, although they are not entirely immune to being held under suspicion of espionage. Within an urban environment, a pigeon has even greater potential to blend into the local avian population, further compounding detection. The latter presumably factored into the use of pigeons to clandestinely smuggle drugs, defeating even the most sophisticated of walls.
Furthermore, pigeons provide an asymmetric tool available for hybrid warfare purposes. The low-cost, low-technology use of pigeons to transport information or potentially small amounts of chemical agents — or even coded cyber weapons — makes them a quick and easy asset to distribute among a civilian population for wider military purposes. During World War II, the British Confidential Pigeon Service of MI14(d) dropped baskets of homing pigeons behind enemy lines for espionage purposes, gathering invaluable military intelligence in the process from a wide array of French, Dutch, and Belgian civilians. Even as a one-way means of communication, the pigeon proved an invaluable military asset.
Via Schneier, who reminds that we have an RFC.