I have it from an article ;-) According to Wikipedia, Plinius the elder mentiones in his "naturalis historia", that Decimus Iunius Brutus Albinus held contact with Aulus Hirtius, while he was besieged by Marcus Antonius at Mutina, by sending pigeons. Sextus Julius Frontinus mentiones Caesar using them during his conquest in Gallia. It says, that the Persians used them first.
I will find you better sources. Look for my next post.
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