Re: How do they exchange the racing pigeons?
You have correctly identified a significant problem with private pigeon exchange.
This is why work is in progress to develop public pigeon exchange. The major problem to be solved is signing the pigeons: tattoos require the pigeon to be plucked and then time is needed to replace feathers. Obviously, something better is needed.
Once the scheme is perfected, perfect forward privacy will be assured by the use of session pigeons carried by the public pigeons.