1604: How Birds Have Such a Vocal Range: Syrinx | May 6, 2019

Most of the sounds produced by human speech is done through the tongue, lips, nasal tract, larynx, and teeth. Birds have very few of those physical attributes, save tongue, and yet are able to produce an amazing range of notes. This is because, instead of a larynx, they have a syrinx, which is a type of larynx, but rather than being at the top of the esophagus, it is at the bottom. They also have two sets of vocal folds (human have only one) and are able to manipulate them both separately at the same time. This is how they are able to produce much of the range of sounds they can, though their tongue and beaks do play a factor as well.

For even more on 'syrinx', see here.