system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do carrier pigeons become trained to fly from place to place

Seriously did someone tie a bit of string to their foot and walk from place to place till they learned? How did the senders know that the pigeons were going to the right place?
gpt
Historically, pigeons carried messages only one way, to their home. They had to be transported manually before another flight. However, by placing their food at one location and their home at another location, pigeons have been trained to fly back and forth up to twice a day reliably, covering round-trip flights up to 160 km (100 mi).[2] Their reliability has lent itself to occasional use on mail routes, such as the Great Barrier Pigeongram Service established between Auckland, New Zealand and Great Barrier Island in November 1897.[3]


Carrier Dove, clipper ship
With training, pigeons can carry up to 75 g (2.5 oz) on their backs. The German apothecary Julius Neubronner used carrier pigeons to deliver urgent medication.[4] In 1977 a similar carrier pigeon service was set up for the transport of laboratory specimens between two English hospitals. Every morning a basket with pigeons was taken from Plymouth General Hospital to Devonport Hospital. The birds then delivered unbreakable vials back to Plymouth as needed.[5] The 30 carrier pigeons became unnecessary in 1983 because of the closure of one of the hospitals.[6] In the 1980s a similar system existed between two French hospitals located in Granville and Avranche.[7]