any of 12 species of solitary marsh birds of the subfamily Botaurinae, family Ardeidae (order Ciconiiformes), allied to the herons (heron) (subfamily Ardeinae) but with shorter neck and stouter body. Most bitterns bear a camouflage pattern—streaks of variegated brown and buff—which enables them to escape detection by standing upright with bill pointed upward, imitating the reeds and grasses of their habitat. They feed upon fish, frogs, crayfish, and other small swamp and marsh animals, which they spear with their sharp-pointed bills. Bitterns occur almost worldwide. There are four species of Botaurus and eight species of Ixobrychus.

Bitterns of the genus Botaurus, occurring mainly in temperate regions, are large, and the sexes look alike. In spring the male utters booming calls audible for a considerable distance. The female undertakes nesting duties; assembling a crude mass of vegetation near water level, she lays four to six brownish eggs. The largest member of the genus is the Eurasian bittern (B. stellaris), to 75 centimetres (30 inches), ranging from the British Isles to southeastern Asia and occurring also in South Africa. The American bittern (B. lentiginosus), known locally as “stake driver” or “thunder pumper,” is slightly smaller. Other forms are the Australian bittern (B. poiciloptilus) and the South American, or pinnated, bittern (B. pinnatus).

Bitterns of the genus Ixobrychus are small (30 to 40 cm, or about 12 to 16 in.). The sexes are unlike in appearance and share in the nesting duties. As many as 10 white, bluish, or greenish eggs are laid in a neat nest placed well above water level, sometimes in a tree. Superficially alike are the least bittern (I. exilis), of America; the little bittern (I. minutus), of Eurasia, Africa, and Australia; and the Chinese little, or yellow, bittern (I. sinensis). Rather similar are the variegated, or stripe-backed, bittern (I. involucris), of South America; the African dwarf bittern (I. sturmii); and, in southeastern Asia, Schrenk's little bittern (I. eurhythmus) and the cinnamon little, or chestnut, bittern (I. cinnamomeus). Somewhat larger is the black mangrove bittern (I. flavicollis), of southeastern Asia and Australia. This species shows plumelike development of the crown and neck feathers and is sometimes separated as Dupetor. For information on tiger bitterns, or tiger herons, see heron.

very bitter-tasting solution that remains after evaporation and crystallization of sodium chloride (table salt) from brines (brine) and seawater. It contains in concentrated form the calcium and magnesium chlorides and sulfates, bromides, iodides, and other chemicals originally present in the brine. It is a commercial source of magnesium compounds—magnesium sulfate (Epsom salts), magnesium chloride, and magnesium bromide. The chemical term is a modification of bitter.