Where mammals have a single set of Vocal Folds in the larynx of their trachea that mean they can only ever bark, moo, yelp, or sing with one voice using their vocal folds, the “voicebox” of birds is further down their respiratory system at the place where the trachea branches into bronchi. Birds have a syrinx rather than a larynx, which allows them to create two tones at once.

Here is an old world flycatcher, the European Robin (Erithacus rubecula) not to be confused with the American Robin (a type of thrush, Turdidae) demonstrating what is possible with syrinx. Listen out for his neighbours calling at the points in the video when he stops singing. It’s impossible to know who sang first, maybe he’s replying, or maybe it’s them calling back to him.

For Rush fans, yes, that is the reference!