Cantus Arcticus is a three movement symphonic work which makes extensive use of birdsong recorded in the Arctic. This piece is not merely a charming experiment. It illustrates that while we think of nature as containing music, the sounds of nature are either not truly music in and of themselves, or calling them music requires a redefinition of the word "music" itself. Unlike the oeuvre of John Cage, who seems tickled to make a significant percentage of his output more a commentary on how far it is possible or not possible to stretch the definition of what we call music, while producing mostly sounds that no one wishes to hear while in the mood for something recognizeable as music, Cantus Arcticus is indeed wonderful music in its own right. But what it also does is illustrate that while we frequently refer to birdsong as being musical, anyone can quickly recognize that without the orchestra, we would not call this music. The sounds of nature are beautiful, but wild. Music is primarily a human concept, and symphonies are highly ordered, being designed rather than emergent.
When the birdsong is sparse, the harmonies of the orchestra are not apparently strongly tonal. Rautavaara thus expresses the wild beauty of nature. When the birdsong is thick and sonorous, the orchestra reduces to simple, tonal harmonies, since it is no longer needed to express nature's wildness; the birds do it themselves. Indeed, some of the tweets towards the end of the piece would probably be interpreted as electronic noise if one were to hear them in some other piece on the radio, a CD or MP3.
Rautavaara went beyond Messaien's use of human-designed instruments to evoke the sounds of birds, instead going directly to the source itself, and produced what he calls a concerto for birdsong and orchestra. And it seems much less pretentious, with his technique for incorporating the two letting birdsong be nature's beautiful noise, and human music be human music.
A side note, the conductor was Finnish composer and conductor Leif Segerstam, who I had the privilege of hearing when he visited Chicago in 1997. He loves making a big deal of Sibelius' dynamic techniques for brass (e.g.: sforzando, subito piano, then a big crescendo), and incorporated them strikingly in Sibelius' youthful and patriotic opus, Finlandia.