The marking of carrier pigeons

The carrier pigeon is a variety of bird belonging to the species Pigeon biset (Columba livia), specially selected to travel long distances to deliver messages.
The movements of carrier pigeons were a common means of communication and considered very reliable until the end of World War I. Breeders of this breed of bird are called pigeon fanciers, and the messages sent are called pigeon telegrams.
For the military, carrier pigeons remained an effective means of transmission: there was little risk of interception (a pigeon is harder to target than a balloon), they were inexpensive (a pigeon is light to carry and easy to feed), almost invisible (how can you recognize a military pigeon in the sky?) and completely silent (compared to an airplane). If necessary, a pigeon could also carry microfilms. Or even blood samples, as was the case between L’Île-d’Yeu and Les Sables-d’Olonne until the end of the 20th century!