UPDATE: It occurs to me occasionally and I forget to mention it, that this is probably the bird most Brits think of when they hear the song “Rockin’ Robin”…
But…I suspect the guy who wrote the song, Leon René, was actually thinking of the American Robin (Turdus migratorius), which is like a British Blackbird but with a redbreast, like a European Robin (I say red, it’s orange in both Robins). Moreover, the American’ Robin’s song is much closer to the refrain Tweet, tweedle-lee-dee in hit, than the song of the European Robin. One more thing, check out the cover artwork of the record by original Rockin’ Robin artist, Bobby Day, he’s got macaws, parrots, but no sign of a Robin, American, European or otherwise as far as I can see.
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!