There's a quotation from Messiaen that reads: "I doubt that one can find in any human music, however inspired, melodies and rhythms that have the sovereign freedom of bird song."
While Messiaen found birds to be "sovereign" in their creative capacity, he also said they are "the closest to us, and the easiest to reproduce". I should assert that the only man-made music ever, perhaps, to come close to birdsong is Gregorian chant. This music, the music proper to the liturgy of the Roman Catholic Church, manifests the same flexibility of both melody and rhythm. There is even evidence to suggest that the Gregorian melodies we have written down were the basis, in fact, of improvisation – which of course further reminds us of the sounds of the natural world.
Again, we note that in music inspired by birds we find an opportunity to explore the natural sounds around us, which is another reason why there is such vitality and depth in all this music.