Darter - Distribution and Ecology

Distribution and Ecology

Darters are mostly tropical in distribution, ranging into subtropical and barely into warm temperate regions. They typically inhabit fresh water lakes, rivers, marshes, swamps, and are less often found along the seashore in brackish estuaries, bays, lagoons and mangrove. Most are sedentary and do not migrate; the populations in the coolest parts of the range may migrate however. Their preferred mode of flight is soaring and gliding; in flapping flight they are rather cumbersome. On dry land, darters walk with a high-stepped gait, wings often spread for balance, just like pelicans do. They tend to gather in flocks – sometimes up to about 100 birds – and frequently associate with storks, herons or ibises, but are highly territorial on the nest: despite being a colonial nester, breeding pairs – especially males – will stab at any other bird that ventures within reach of their long neck and bill. The Oriental Darter (A. melanogaster sensu stricto) is a Near Threatened species. Habitat destruction along with other human interferences (such as egg collection and pesticide overuse) are the main reasons for declining darter populations.

Darters feed mainly on mid-sized fish; far more rarely, they eat other aquatic vertebrates and large invertebrates of comparable size. These birds are foot-propelled divers that quietly stalk and ambush their prey; then they use their sharply pointed bill to impale the food animal. They do not dive deep but make use of their low buoyancy made possible by wettable plumage, small air sacs and denser bones. On the underside of the cervical vertebrae 5-7 is a keel, which allows for muscles to attach to form a hinge-like mechanism that can project the neck, head and bill forward like a throwing spear. After they have stabbed the prey, they return to the surface where they toss their food into the air and catch it again, so that they can swallow it head-first. Like cormorants, they have a vestigial preen gland and their plumage gets wet during diving. To dry their feathers after diving, darters move to a safe location and spread their wings.

Predators of darters are mainly large carnivorous birds, including passerines like the Australian Raven (Corvus coronoides) and House Crow (Corvus splendens), and birds of prey such as marsh harriers (Circus aeruginosus complex) or Pallas's Fish Eagle (Haliaeetus leucoryphus). Predation by Crocodylus crocodiles has also been noted. But many would-be predators know better than to try to catch a darter. The long neck and pointed bill in combination with the "darting" mechanism make the birds dangerous even to larger carnivorous mammals, and they will actually move towards an intruder to attack rather than defending passively or fleeing.

They usually breed in colonies, occasionally mixed with cormorants or herons. The darters pair bond monogamously at least for a breeding season. There are many different types of displays used for mating. Males display to attract females by raising (but not stretching) their wings to wave them in an alternating fashion, bowing and snapping the bill, or giving twigs to potential mates. To strengthen the pair bond, partners rub their bills or wave, point upwards or bow their necks in unison. When one partner comes to relieve the other at the nest, males and females use the same display the male employs during courtship; during changeovers, the birds may also "yawn" at each other.

Breeding is seasonal (peaking in March/April) at the northern end of their range; elsewhere they can be found breeding all year round. The nests are made of twigs and lined with leaves; they are built in trees or reeds, usually near water. Typically, the male gathers nesting material and brings it to the female, which does most of the actual construction work. Nest construction takes only a few days (about 3 at most), and the pairs copulate at the nest site. The clutch size is two to six eggs (usually about 4) which have a pale green color. The eggs are laid within 24–48 hours and incubated for 25 to 30 days, starting after the first has been laid; they hatch asynchronously. To provide warmth to the eggs, the parents will cover them with their large webbed feet, because like their relatives they lack a brood patch. The last young to hatch will usually starve in years with little food available. Bi-parental care is given and the young are considered altricial. They are fed by regurgitation of partly digested food when young, switching to entire food items as they grow older. After fledging, the young are fed for about two more weeks while they learn to hunt for themselves.

These birds reach sexual maturity by about 2 years, and generally live to around 9 years. The maximum possible lifespan of darters seems to be about 16 years.

Darter eggs are edible and considered delicious by some; they are locally collected by humans as food. The adults are also eaten occasionally, as they are rather meaty birds (comparable to a domestic duck); like other fish-eating birds such as cormorants or seaducks they do not taste particularly good though. Darter eggs and nestlings are also collected in a few places to raise the young. Sometimes this is done for food, but some nomads in Assam and Bengal train tame darters to be employed as in cormorant fishing. With an increasing number of nomads settling down in recent decades, this cultural heritage is in danger of being lost. On the other hand, as evidenced by the etymology of "anhinga" detailed above, the Tupi seem to have considered the Anhinga a kind of Bird of Ill Omen.