In the creatures of earth human and birds are only can able to produce human language sounds.
Naturally many birds have the potential to mimic the various sounds that hear. In that, parrots, cockatoos, cockatiels from parrot family and mynah birds and corvids from the crow family are famously known birds. These all have the ability to reproduce sounds what they hear.
But parrots have most perfect vocal match with humans. So its imitate perfectly as human speech.
The Parrot has a very developed vocal system, which enables it to imitate numerous sounds. Thus, it can reproduce the ringing of a telephone, a melody, or words. That is how it gives the impression that it can talk.
In a study Erich Jarvis, Duke University neuroscientist and vocal learning expert, recently published a study in Plos One explaining why. Any bird that’s vocal learner has a part of the brain devoted to this, called the ‘song system has two layers – an inner ‘core’, common to all avian vocal learners, and an outer ‘shell’, which is unique to parrots to be such expert mimickers ( though he hasn’t figured out exactly how it works yet )
We think parrot just imitate what we say. But not actually like that.
“ Parrots who talk understand what they’re saying if they are taught appropriately”, pepperberg says. For example , a bird taught to identify favourite foods knows precisely what they imply when they ask for them. For example , Waldo, a 21 year old African Grey Parrot who has been part of the band Hatebeak for 12 years (what started as a laugh has become a successful venture), loves snacking on bananas and crackers. Because Drummer Blake Harrison told vice, “we got him dehydrated banana chips, and he pieced it together and called them ‘ banana crackers’ on his own. Its a little creepy”. By the end of his life, Pepperberg’s Alex had learned to identify 50 objects, 7 colors, 6 shapes. (net source)
Thus the parrot, not only imitating its also able to understand of words. So it can speak.