For those that do not wish to use the normal means of in game delivery, birds that are fed and kept happy by singing songs to them that some times perch upon liked ones shoulders, and healthy trees keeps them socially happy which increases mood. Mood levels could determine time it takes to send a message and on the way possibly they can pick up an item depending on distance. Items such as, worms or something seemingly insignificant, but these worms could allow for exp growth of a bird and with levels these randomly appearing worms which supply exp in small amounts, which gives them value to those who want to develop their birds skills in being a .. bird. As a bird grows it may be able to gain small things such as an extra tail feather which shows by appearance that it has been worked on, or larger wing span to represent distance flown or thicker feathers to represent more small distance flights. pidgeons with a gold halo, and sparkles following it, or hawk with dripping blood and a dark aura? Both seem extremely attractive as an idea but the work toward it would influence people to send things long distances and patiently wait for things sent as it would sometimes give worms which of course any bird owner would love to get their hands on as it will level their bird and allow for the bird to reflect the care put in to it. I believe this could also act as an in game tool for trade via birds, which would not stop auctions or any thing because of the time it takes to arrive but when it arrives it provides a trade with expected currency values from and to, and changes of original trade would have to be sent back via notes or what ever it may be it sounds like a fun idea let me know what you think this is just brain storming!