A recent study discovered that bats jam each other's sonar by emitting siren-like short calls; this interferes with the echolocation of their competition. This is the first time a species has been observed jamming other members of its own species (except, I suppose, for the case of navy engineers jamming enemy's sonar). In the past, however, others have found that certain prey species, such as moths, jam the sonar of predators. The "jamming" basically acts as an acoustic blindfold, confusing or blocking the signals captured by echolocation.

I have found that the best written description of echolocation I've ever seen comes from Calvin and Hobbes (although Calvin's understanding of bats is far from perfect!)