Artist Jessica Rylan made an interesting sound installation that features two types of bird sounds. Here's how she describes how she made the sounds: I did it by modulating the supply voltage, a popular circuit-bending trick. One bird chirps more frequently, sometimes sounding like a seagull. The other bird sings less frequently, but it has a long graceful warble like something that likes to stay hidden deep in the forest. Every once in a while, one of them makes an odd noise, sometimes a gasp and sometimes a ringing cell phone. This page on her website describes the project in more detail. I like the idea of a nature diorama in which you hear the birds or animals, but don't necessarily see them.