Anhinga (Darter or Snakebird)
The darters or snakebirds are mainly tropical waterbirds in the family Anhingidae. 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.
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 a 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 colour 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 colour between yellow, red or brown seasonally. The young hatch naked, but soon grow white or tan down.
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.
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 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.
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 and 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.