No matter where in the world it goes, it knows where its nest is, so it never gets separated from its Trainer.
(From Bulbapedia) Tranquill is an avian Pokémon that is gray with lighter gray and black markings. Its beak and eyes are yellow, and there is dark pink marking above its beak that curves back over the eyes. It has a black tuft of two feathers on its head and a petal-like arrangement of feathers on the nape of its neck. There is a black stripe on each of its light gray, black-tipped wings. Each pink foot has black talons. It possesses a long tail feather that is tipped in black. No matter how far a Tranquill travels, it is always able to return to its Trainer. It is said to live in deep forests, where there is a peaceful place without war.
(From Bulbapedia) Judging from its Pokédex entries, Tranquill may be based on a dove, often used as a symbol of peace, and possibly a homing pigeon, a domestic pigeon bred to find its way home over extremely long distances. Its design is similar to that of the greater roadrunner or secretarybird.
(From Bulbapedia)Tranquill is a combination of tranquil (calm or unstressed) and quill (a feather from the tail of a bird). It also involves quail.