To communicate with each other through dense vegetation American Bitterns use low-frequency calls, which carry farther than higher-pitched sounds. During breeding season the males make a bizarre, resonant three-syllable pump-er-lunk call with a liquid quality; females may respond with a similar but quieter sound. The male’s call is preceded by clacking and gulping. To accomplish the pump-er-lunk sound, the male inflates his esophagus by way of almost violent body contortions—opening and closing his bill as if lunging for flying insects—and then uses the stored air to unleash his call. Repeated up to 10 times in succession, the call probably serves as both a territorial signal and an advertisement for mates. When flushed, American Bitterns often emit a hoarse kok-kok-kok or nasal haink. Males may give a continual chu-peep call during mating.
Bitterns clack the mandibles of their bills together to make sounds preceding their territorial call.