Bitterns

views updated

Bitterns

Bitterns are about 12 species of wading birds in the subfamily Botaurinae of the family Ardeidae, which also includes herons and egrets. There are two genera: four species of the relatively large and stocky true bitterns (Botaurus spp.), and eight species of the much smaller and more slender, least bitterns (Ixobrychus ).

Bitterns have brown-and-black, vertically streaked plumage, which renders them well camouflaged in their marshy or reed-fringed habitats. Male and female bitterns have identical plumage. When a potential predator is in the vicinity, bitterns will try to blend in with their surroundings by extending their neck and bill upright, compressing their brownish-streaked breast plumage, and facing the intruder. Bitterns may also sinuously wave their body to emulate the movements of the surrounding, wind-blown reeds and bulrushes.

Bitterns are unobtrusive animals, and many people are unaware of the presence of these animals, even in marshes where they are breeding. Bitterns fly with a slow wingbeat, low over the tops of the marsh vegetation, and then suddenly drop down out of sight to land.

Bitterns mostly eat fish, but they also take aquatic invertebrates, snakes, frogs, baby birds, and small mammals when these are available. Bitterns slowly and deliberately stalk their prey, and then capture their victim by quickly spearing it with a rapid thrust of the beak.

Bitterns usually nest on rough platforms that they construct out of sticks or marsh vegetation. The nest may be placed in a concealed place on the ground, or

in a shrub or low tree. Bitterns lay three to six eggs, which are incubated by the female, who is also mostly or totally responsible for rearing the brood.

Species of bitterns

The American bittern (Botaurus lentiginosus ) breeds in freshwater and brackish marshes over most of the temperate zone of North America, and as far south as central Mexico. Most North American populations migrate to the southern United States and Central America to spend their non-breeding season. American bitterns mostly eat fish, but they will also prey on other appropriately sized, aquatic prey. Male American bitterns have a distinctive song in the springtime, when they are establishing a breeding territory and attempting to attract a mate. This booming call can be heard over a distance of several miles, and more or less sounds like a pumping, “ong-ka-chonk.” Some local names of the American bittern reflect its call: “thunder pump” and “stake driver.” Other species of Botaurus occur in nonoverlapping

KEY TERMS

Marsh— A type of productive wetland that is dominated by tall, emergent plants, such as reeds, bulrushes, and cattails.

Superspecies— A complex of closely related groups of organisms that are geographically, ecologically, and morphologically distinct, but are nevertheless considered to be the same species. The seasise sparrows are a superspecies, in which many of the various subspecies were formerly believed to be separate species.

Wader— This is a general term for various long-legged, long-necked, long-beaked, short-tailed birds of marshes and swamps that stand in shallow water while stalking their prey. Waders include species in the heron family, as well as birds in other families, such as storks, ibises, flamingos, spoonbills, and cranes.

ranges on other continents. These species are all rather similar, and although considered to be taxonomically distinct, they form a closely-related, “superspecies.” The other species of bitterns include the Eurasian bittern (Botaurus stellaris ) of Europe and Asia, the Australian bittern (B. poiciloptilus ), and the South American bittern (B. pinnatus ).

The least bittern (Ixobrychus exilis ) is the smallest species of heron in North America. This secretive species breeds widely in freshwater and coastal marshes in the eastern United States and southeastern Canada. Least bitterns also breed in a disjunct, western range in California and Oregon, and south to Brazil, Paraguay, and northern Argentina. Northern populations of the least bittern migrate to northern Mexico and Baja California for the winter. The least bittern mostly feeds on small fish, which it stalks patiently and then spears with its bill.

The various species of Ixobrychus also have non-overlapping ranges, and also form a closely-related “superspecies.” The species include the little bittern (I. minutus ) of Europe, Asia, Africa, and Australia, and the Chinese little bittern (I. sinensis ) of eastern Asia.

Conservation of bitterns

Habitat losses associated with the drainage of wetlands for agricultural and residential developments are the most important threats to bitterns in North America and elsewhere. Pollution may also be significant in degrading habitat in some regions.

As a result of these and other stressors, the populations of both American bitterns and least bitterns are widely acknowledged as having declined substantially in North America. There is significant concern about the population status of both species in most parts of their ranges in the United States and Canada.

Like so many other species that require wetlands as habitat, the survival of bitterns can only be ensured by caring for the ecosystems of which they are an integral part. In this case, the key is the conservation and protection of wetlands.