The thesis about human speech occupying less crowded spectrum is well aligned with a book called "The Great Animal Orchestra" (https://www.amazon.com/Great-Animal-Orchestra-Finding-Origin...).

That author details how the "dawn chorus" is composed of a vast number of species making noise, but who are able to pick out mating calls and other signals due to evolving their vocalizations into unique sonic niches.

It's quite interesting but also a bit depressing as he documents the decline in intensity of this phenomenon with habitat destruction etc.