2 way encryption carrier pigeons.
How many pigeons do you need to actually make this work? You need pigeons to send key exchange prior to sending the message. Also pigeons to proof of receive the message. you might need at least 12 pigeons to make this work. Also you need to be able to check if the pigeons have been compromise, such as pigeons from another source which pretend to me the sender or receiver. Also this will be seasonal, you will not be able to transfer any messages during the pigeon hunting season.
You forgot about the pigeons that check if the receiver is ready to receive.
