Almost all species of parrots are capable to say though some of them manage it better than others. Partly the secret of garrulity of these birds is that they are by nature inclined to a gregarious way of life and, the deprived opportunities to communicate to other parrots, do by members of the pack of people, trying to find with them a common language.
The organs of articulation of birds are somewhat similar to human. Their lower throat performs the same function, as vocal chords at people and process of pronouncing sounds at the person and a bird has many common features. Special difference of parrots is their ability to remember and reproduce new sounds. I.e. if other birds use the general sound signals and do not study new songs as they do not need it, then parrots try to remember the human speech and to learn to imitate it to become full members of pack.
Frequency of sounds which parrots are capable to publish is brought usually closer to the average frequency of a female voice, and therefore some species of these birds are often not capable to simulate the men's speech rather plausibly. Movable by natural aspiration to imitation, they can reproduce also other sounds, including a melody of a call of phone or dog bark. It is partly caused by the fact that in the native habitat the young parrots try to say the same sounds, as the senior birds, but in the human house they are deprived of society similar and forced to imitate everything that is heard.
It is interesting that some parrots can speak with a definite purpose: for example, so they try to attract attention the person or expect to receive an award. These birds are unusually bright, and some of them can remember the whole offers or even short texts. Relationship of cause and effect at them is developed very well therefore parrots quickly notice reaction of people to any given words and remember it. Thanks to a number of the made experiments the scientists managed to find out that parrots are capable not only to pronounce words, but even to understand what they mean. So, gray African parrots who are famous for the ability faultlessly to imitate the human speech up to intonations, can remember names more than one hundred objects and even to understand what word what designates. If to give to such parrot delicacy for each thing which is correctly called by it, he will very quickly learn to speak.