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The darters, anhingas, or snakebirds are mainly tropical waterbirds in the family Anhingidae,which contains a single genus, Anhinga. There are four living species, three of which are very common and widespread while the fourth is rarer and classified as near-threatened by the IUCN. The term snakebird is usually used without any additions to signify whichever of the completely allopatric species occurs in any one region. It refers to their long thin neck, which has a snake-like appearance when they swim with their bodies submerged, or when mated pairs twist it during their bonding displays. "Darter" is used with a geographical term when referring to particular species. It alludes to their manner of procuring food, as they impale fishes with their thin, pointed beak. The American darter (A. anhinga) is more commonly known as the anhinga. It is sometimes called "water turkey" in the southern United States; though the anhinga is quite unrelated to the wild turkey, they are both large, blackish birds with long tails that are sometimes hunted for food.
Description & appearance
Anhingidae are large birds with sexually dimorphic plumage. They measure about 80 to 100 cm (2.6 to 3.3 ft) in length, with a wingspan around 120 cm (3.9 ft), and weigh some 1,050 to 1,350 grams (37 to 48 oz). The males have black and dark-brown plumage, a short erectile crest on the nape and a larger bill than the female. The females have much paler plumage, especially on the neck and underparts, and are a bit larger overall. Both have grey stippling on long scapulars and upper wing coverts. The sharply pointed bill has serrated edges, a desmognathous palate and no external nostrils. The darters have completely webbed feet, and their legs are short and set far back on the body.
There is no eclipse plumage, but the bare parts vary in color around the year. During breeding, however, their small gular sac changes from pink or yellow to black, and the bare facial skin, otherwise yellow or yellow-green, turns turquoise. The iris changes in color between yellow, red or brown seasonally. The young hatch naked, but soon grow white or tan down.
Voice, singing & call
Darter vocalizations include a clicking or rattling when flying or perching. In the nesting colonies, adults communicate with croaks, grunts or rattles. During breeding, adults sometimes give a caw or sighing or hissing calls. Nestlings communicate with squealing or squawking calls.
Distribution & habitat
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
Hunting & food
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 which 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. Darters go through a synchronous moult of all their primaries and secondaries making them temporarily flightless, although it is possible that some individuals go through incomplete moults.
Breeding & mating
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 three at most), and the pairs copulate at the nest site. The clutch size is two to six eggs (usually about four) 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.
Chicks, juveniles & raise
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 two years, and generally live to around nine years. The maximum possible lifespan of darters seems to be about sixteen 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.
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