Music composed by birds. Sort of. I once heard a lecturer* on modern classical music describe an analogous experiment in which a stripper cast her garments onto wires and musicians interpreted the results. I imagine the birds have done better.

*A really oddly stilted fellow, the lecturer: he would cast octogenarian-style aspersions on the "Grateful Stones" then go right on to describe a piano concerto performed with the composer inside the piano, scraping a piece of chalk on anything that would make a noise.