Bats are a difficult group of mammals to study because they:

1. are nocturnal,
2. fly relatively fast,
3. produce sound above the range of human hearing,
4. roost in cavities, cracks, and crevices that are difficult to find, and/or
5. their coloration allows them to blend in with the tree foliage or bark on which they rest during the day.

It was not until the late 1930's that scientists discovered that some bats produce ultrasonic signals (sound above the range of human hearing) to locate and intercept insect prey in flight. Bats emit these high-frequency calls and listen for returning echoes to determine distance and direction of various objects in their path. Echo-location, as the finding was termed, allows bats to fly in complete darkness while searching for tiny insects and to avoid obstacles like overhanging tree branches. One important outcome of this discovery was that it opened up a whole new way of studying bats in the wild by using electronic tools for ultrasound detection. Observers can listen for bats just as bird watchers have listened for birds for many years past.