Parrots play social games that involve very complex vocalization. They’re pretty intelligent and have very flexible vocal machinery that can make a very wide range of sounds. The result of this is that our speech is within the range of sounds they can make, which allows them to learn to imitate us, and they’re smart enough to use our words in a reasonably coherent fashion. But there’s nothing special about human speech. They can also learn to imitate dogs, traffic noises, ringtones, music, etc.
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