The deep booming mating call of the male great bittern is called a boom.
During the breeding season, a male great bittern produces a remarkably low, resonant sound that can carry across wetlands. The call is often compared with the sound of blowing across the top of a bottle. It advertises the male’s presence and helps attract females while warning other males away from his territory.
The bird creates the boom with its specialized vocal anatomy and greatly expanded oesophagus, which acts as a resonating chamber. A male may repeat the call several times, especially during the breeding season. The sound is much easier to hear than the well-camouflaged bird is to see.
The great bittern is a heron, not a crane or a bittern-like mammal. Its cryptic brown plumage and habit of standing motionless among reeds make visual identification difficult, so the booming call is one of the species’ most useful field clues.