Abstract: A major portion of Olivier Messiaen's music imitates birdsongs. Even for a composer known for his stylistic independence, this 'bird style' seems unusually idiosyncratic. His worldwide sojourns to transcribe birdsongs appertain more to ornithology, it would seem, than to music composition, and critics have found his birds easy targets for their barbs. In order to better understand Messiaen's bird style, I examine both the pervasive critical bias against nature representation in music and the bird style's relationship to art and science. I show that Messiaen's early works contain far more birdsongs than has previously been recognized and, from my research with Messiaen's notebooks at the Bibliothèque nationale de France, show that he used a sound recording to transcribe the North American birdsongs in his most famous orchestral bird piece, Oiseaux exotiques (1956). With spectograms made from this recording, I offer the first detailed analysis of the accuracy of Messiaen's transcriptions. I examine both the language of Messiaen's bird style in its gestural, harmonic, and formal structures from 1929 to 1956 and its cultural milieux in music, literature, painting, science, politics, and theology. I argue that the 'early bird style' expresses joy in response to suffering; that in the early years of the Cold War, the birds convey peace; and that with the advent of his 'late bird style,' which relies on direct transcriptions, his birds signify creation as a conduit to divine wisdom. These meanings reflect the Catholic Church's crusade against modernism, which championed Thomas Aquinas's reconciliation of science with faith. The two pillars of science and faith have long defined Messiaen's output, with science manifest in his ornithologically-informed bird style and faith manifest in his many works on Catholic themes. I show how the bird style combines science and faith through Gothic spirituality. By using sensible reality (birdsong) to approach a higher, divine reality (symbolized by symmetry, prime numbers, and other means), Messiaen creates an aesthetic of 'double realism' to effect a Neoplatonic spirituality associated with 13th-century France, Gothic architecture, and scholasticism. Although Messiaen's mimesis appears unprecedented, in fact it replays the music of the spheres.