Brutus and the Siege of Modena
Ancient Greeks and Romans used homing pigeons to send all types of messages. As early as the 8th century B.C., Greeks used homing pigeons to send news of the Olympic champions. We also know of a Roman magistrate in the 4th century who sent a pigeon to merely say that he'd be late coming home from the theater.
But perhaps the greatest ancient use of carrier pigeons was in 44 B.C., during Marc Antony's siege of Modena (a city in Northern Italy, then known as Mutina). Marcus Junius Brutus successfully defended the city, thanks in part to his use of pigeons to send messages to his allies, Decimus and Hirtius.
Pliny the Elder actually writes about Brutus using pigeons, saying, "Of what avail were sentinels, circumvallations or nets obstructing the rivers, when intelligence could be conveyed by aerial messengers?"