The American Bitterns camouflaged plumage and secretive behavior help it to blend into the thick vegetation of its freshwater wetland habitat. It is heard much more often than it is seen; its call is unmistakable and has given rise to many evocative colloquial names, such as "thunder pumper". Deep, resonant pump-er-unk, pump-er-unk calls mainly at dawn, dusk, and night time, but also during the day in the early mating season.
American Bitterns breed mainly in freshwater marshes with tall vegetation. You can find them in wetlands of many sizes and kinds, typically less densely vegetated and shallower than wetlands used by the Least Bittern. In winter they move to areas where water bodies don't freeze, especially near the coast, where they occasionally use brackish marshes. Managed wetlands such as wildlife refuges seem to be important for wintering American Bitterns. Wintering birds may also forage in dry grasslands and other terrestrial habitats.
American Bitterns eat insects, crustaceans, fish, amphibians, reptiles, and small mammals. Their most common insect prey include water striders, giant water bugs, water beetles, water scorpions, grasshoppers, and especially dragonflies, which the birds sometimes manage to capture in midair. Frequently consumed fish include eels, catfish, pickerel, sunfish, suckers, perch, killifish, and sticklebacks. Rayfish, crabs, frogs, tadpoles, salamanders, garter snakes, water snakes, and meadow voles round out the diet. American Bitterns usually forage in dim light, at shorelines and the fringes of vegetated areas. A foraging bird may sway its neck, perhaps to see past glare from the surface of shallow water, or to warm up its muscles for a quick strike. A characteristic strategy is to stand stock-still with bill held horizontal, gradually aiming the bill downward with nearly imperceptible movements—until, with a sudden darting motion, the bittern seizes the prey in its bill, bites or shakes it to death, and swallows it head first. Indigestible parts of prey animals are regurgitated as pellets.