during the 19th-century (1870–71) franco-prussian war, besieged parisians used carrier pigeons to transmit messages outside the city; in response, the besieging prussian army employed hawks to hunt the pigeons. the french military used balloons to transport homing pigeons past enemy lines. microfilm images containing hundreds of messages allowed letters to be carried into paris by pigeon from as far away as london. more than one million different messages travelled this way during the four-month siege. they were then discovered to be very useful, and carrier pigeons were well considered in military theory leading up to world war i.