The homing pigeon is a variety of domesticated Rock Pigeon that has been selectively bred to be able to find its way home over extremely long distances. Homing pigeons are called carrier pigeons when they are used to carry messages.
Most researchers believe that the homing ability is based on a "map and compass" model, with the compass feature allowing birds to orient and the map feature allowing birds to determine their location relative to a goal site (home loft). The compass mechanism appears to rely on the sun, the map mechanism is believed to rely on the ability of birds to detect the Earth's magnetic field but after nearing their home lofts, in areas they have previously visited, pigeons probably are guided by visual landmarks.
People often have the misunderstanding that the pigeons know where to deliver the messages. The fact is that they can only go back to one "mentally marked" point that they have identified as their home. So "pigeon mail" can only work when the sender is actually holding the receiver's pigeons.
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