Sean Carroll in Preposterous Universe:
Many characteristics go into making human beings special — brain size, opposable thumbs, etc. Surely one of the most important is language, and in particular the ability to learn new sounds and use them for communication. Many other species communicate through sound, but only a very few — humans, elephants, bats, cetaceans, and a handful of bird species — learn new sounds in order to do so. Erich Jarvis has been shedding enormous light on the process of vocal learning, by studying birds and comparing them to humans. He argues that there is a particular mental circuit in the brains of parrots (for example) responsible for vocal learning, and that it corresponds to similar circuits in the human brain. This has implications for the development of intelligence and other important human characteristics.