There's a nice Boston Globe article from last year that details a then-upcoming concert centered around his "Catalogue d'Oiseaux" ('catalog of birds') piano works and explains how OM based the songs on bird calls that he heard. He actually filled up hundreds of notebooks to chronicle these celestial sounds as a basis for his work. It's no wonder that OM considered himself a ornithology as well as a composer.
See OM explain how he used bird songs to compose his music in this YouTube video & follow OM into the wilderness as he waxes poetic about his favorite singers in the wild in this other video. Learn more about his work through this Boston University site dedicated to him.