I mean, if you release a bird in paris and send it to another city, how does the bird know where to go? Are individual birds specialized to certain towns?
Pigeons try to return to their homes where they were hatched. So to send a bird from Paris to Callais, you had to first bring Callais bird in cage to Paris. There are illustrations of wicker cages used to transport birds for the purpose.
Pigeons were used as late as WWI, here is a photograph of a motorcyclist with pigeon basket on his back, and even converted bus was used as a pigeon loft.
Smithonian magazine has nice article about use of pigeons during WWI.
fyi, you'll find some info on carrier pigeons (aka homing pigeons) in the FAQ
Also, this isn't necessarily a historical question, because carrier pigeons still exist. You might try a pigeon sub (/r/pigeon, maybe /r/petdoves?) or a local racing pigeon association
