Seeing the night with sound

On a summer evening, a bat flickers over a pond, turns sharply, dips and turns again. It is catching insects in near darkness, at speed, without bumping into branches. To us the evening seems quiet. To the bat, it is full of sound.

Shouting and listening

Many bats find their way and their food by echolocation. The U.S. National Park Service explains the principle simply: bats produce sound waves at frequencies above the range of human hearing — ultrasound — and those sounds bounce off objects and return to the bat’s ears.

From the echoes, a bat can build up a picture of its surroundings. A branch, a wall or a moth each returns the sound in its own way. The time it takes for an echo to come back tells the bat how far away something is; changes in the echo tell it whether that something is moving.

It is a little like shouting in a large hall and hearing your voice come back from the far wall — except that bats do it many times a second, and read far more detail from the result.

The feeding buzz

When a bat is just cruising, its calls come at a steady pace. When it detects an insect it wants to eat, the rhythm changes. As the Park Service describes, the bat then produces a rapid series of calls, known as a "feeding buzz".

The faster calls give the bat more frequent updates on exactly where its prey is in the final moments of the chase. Researchers who record bats with special ultrasonic microphones can actually hear when a hunt is reaching its climax.

Every species has its own accent

Echolocation calls are not all alike. According to the Park Service, each species of bat has its own call pattern. That makes recordings useful: by studying the shape and pitch of the calls, scientists can often tell which species are flying in an area, even when they cannot see the bats themselves.

There are also exceptions to the "too high to hear" rule. The spotted bat, Euderma maculatum, which lives in western North America, is faintly audible to people with good hearing.

A hidden world, made audible

Because most echolocation calls are ultrasonic, people can only hear them with help. Bat detectors convert the calls into sounds we can hear. Recordings can also be slowed down: the Park Service presents a spotted bat call played ten times slower so that humans can hear it. Slowed down, the calls turn into chirps, sweeps and clicks — the private language of the night sky.

Bats use their voices for more than navigation. In the Park Service’s recordings, the pallid bat makes both echolocation calls and social calls, used to communicate with other bats.

Why it matters

Echolocation lets bats hunt at night, when many insects are active. It means that every dark garden, field and riverbank may be crossed by animals "seeing" with sound. Protecting bats starts with noticing them — and a summer evening with a bat detector is one of the easiest ways to discover just how busy the night really is.