Why do bats make a high-pitched sound as they fly?

Sound of bats - use high pitched sounds to find their way about. They are nocturnal animals. That is they move about by night. So they have developed their hearing to such an extent that they can find their way by a method known as echolocation. The blind-flying abilities of bats were first studied by Lazzaro Spallanzani (1729-1799). He surgically remove the eyeballs from several bats to prove that they did not need to see to fly. In the 20th Century, biologists using electronic instruments have carried out experiments with bats. They have discovered that bats find out where to go by emitting high frequency sounds and receiving the echoes as they bounce off objects. Most of the sounds have too high a frequency to be heard by the human ear. Bats commonly fly together in groups but apparently they are not confused by the sounds and echoes produced by each other. When hunting in woods and in the rain they are able to discriminate between the faint echoes bouncing off insects and those bouncing off the ground, tree trunks, branches, twigs and raindrops. These tiny flying mammals have been using the equivalent of our modern sonar device for millions of years.