How is possible for pet birds, such...

What we call talking is really mimicry. The bird’s tongue and vocal chords make the bird physically able to produce the sounds, but the bird is merely copying the sounds he hears around him. Myna birds, for example, will make all sorts of jungle sounds when captured because that’s what they have heard. Pet birds who learn to “talk” from their owners do not know what they are saying. They are merely repeating what their owners have taught them.