Schneier - Military Carrier Pigeons in the Era of Electronic Warfare

They have advantages:

Pigeons are certainly no substitute for drones, but they provide a low-visibility option to relay information. 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 author points out that both France and China still maintain a small number of pigeons in case electronic communications are disrupted.

And there's an existing RFC.

from Schneier on Security https://www.schneier.com/blog/archives/2019/01/military_carrie.html