Using microphones to capture the sounds made by animals in their habitat, minus humans and their man-made sounds. These sounds are known as the "biophony" and are used to sense the quality and character of the animals' habitat. Wired has the story.
Author's comment: It's a good thing that animals don't have privacy rights in the USA (yet!), or these scientists would be in jail for unlawful surveillance activities!
(Via Cross-Spectrum Acoustics)