| Darter | |
|---|---|
 | |
| MaleAfrican darter Anhinga rufa | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Suliformes |
| Family: | Anhingidae Reichenbach, 1849[1] |
| Genus: | Anhinga Brisson, 1760 |
| Type species | |
| Plotus anhinga Linnaeus, 1766 | |
| Species | |
Anhinga anhinga | |
 | |
| World distribution of the family Anhingidae | |
| Synonyms | |
Family-level: Genus-level: | |
Thedarters,anhingas, orsnakebirds are mainlytropical waterbirds in thefamilyAnhingidae, which contains a single genus,Anhinga. There are four livingspecies, three of which are very common and widespread while the fourth is rarer and classified asnear-threatened by theIUCN. The termsnakebird is usually used without any additions to signify whichever of the completelyallopatric 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 bondingdisplays. "Darter" is used with ageographical term when referring to particular species. It alludes to their manner of procuring food, as they impalefishes with their thin, pointedbeak. The American darter (A. anhinga) is more commonly known as theanhinga. It is sometimes called "water turkey" in the southern United States; though the anhinga is quite unrelated to thewild turkey, they are both large, blackish birds with long tails that are sometimes hunted for food.[2]
Anhingidae are large birds withsexually dimorphicplumage. 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 longscapulars and upperwing coverts. The sharply pointedbill has serrated edges, adesmognathouspalate and no externalnostrils. The darters have completelywebbed feet, and their legs are short and set far back on the body.[3]
There is noeclipse plumage, but the bare parts vary in color around the year. During breeding, however, their smallgular sac changes from pink or yellow to black, and the bare facial skin, otherwise yellow or yellow-green, turnsturquoise. Theiris changes in color between yellow, red or brown seasonally. The young hatch naked, but soon grow white or tandown.[4]
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 acaw or sighing or hissing calls.Nestlings communicate with squealing or squawking calls.[4]
Darters are mostlytropical in distribution, ranging intosubtropical and barely into warmtemperate regions. They typically inhabitfresh water lakes, rivers, marshes, swamps, and are less often found along the seashore inbrackish estuaries, bays,lagoons andmangrove. Most are sedentary and do notmigrate; the populations in the coolest parts of the range may migrate however. Their preferred mode offlight issoaring andgliding; inflapping flight they are rather cumbersome. On dry land, darters walk with a high-stepped gait, wings often spread for balance, just likepelicans do. They tend to gather in flocks – sometimes up to about 100 birds – and frequently associate withstorks,herons oribises, 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. TheOriental darter (A. melanogastersensu stricto) is aNear Threatened species.Habitat destruction along with other human interferences (such as egg collection andpesticide overuse) are the main reasons for declining darter populations.[2]
Darters feed mainly on mid-sizedfish;[5] far more rarely, they eat otheraquaticvertebrates[6] and largeinvertebrates[7] of comparable size. These birds are foot-propelled divers which quietly stalk and ambush their prey; then they use their sharply pointedbill 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.[8] On the underside of thecervical vertebrae 5–7 is a keel, which allows formuscles to attach to form ahinge-like mechanism that can project the neck, head and bill forward like athrowing 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. Likecormorants, they have a vestigialpreen gland and their plumage gets wet during diving. To dry their feathers after diving, darters move to a safe location and spread their wings.[4] Darters go through a synchronousmoult of all their primaries and secondaries making them temporarily flightless, although it is possible that some individuals go through incomplete moults.[9]
Predators of darters are mainly largecarnivorous birds, includingpasserines like theAustralian raven (Corvus coronoides) andhouse crow (Corvus splendens), andbirds of prey such asmarsh harriers (Circus aeruginosus complex) orPallas's fish eagle (Haliaeetus leucoryphus). Predation byCrocodyluscrocodiles 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 carnivorousmammals, and they will actually move toward an intruder to attack rather than defending passively or fleeing.[10]
They usually breed in colonies, occasionally mixed withcormorants or herons. The darterspair bondmonogamously at least for a breeding season. There are many different types ofdisplays 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.[10]
Breeding is seasonal (peaking in March/April) at the northern end of their range; elsewhere they can be found breeding all year round. Thenests 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 pairscopulate at the nest site. Theclutch size is two to sixeggs (usually about four) which have a pale green color. The eggs are laid within 24–48 hours andincubated 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 abrood patch. The last young to hatch will usually starve in years with little food available.Bi-parental care is given and the young are consideredaltricial. They are fed byregurgitation of partly digested food when young, switching to entire food items as they grow older. Afterfledging, the young are fed for about two more weeks while they learn to hunt for themselves.[11]
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.[12]
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 adomestic duck); like other fish-eating birds such as cormorants orseaducks they do not taste particularly good though. Darter eggs andnestlings are also collected in a few places to raise the young. Sometimes this is done for food, but somenomads inAssam andBengal train tame darters to be employed as incormorant fishing. With an increasing number of nomads settling down in recent decades, thiscultural heritage is in danger of being lost. On the other hand, as evidenced by theetymology of "anhinga" detailed below, theTupi seem to have considered the anhinga a kind of bird of illomen.[4]
ThegenusAnhinga was introduced by the French zoologistMathurin Jacques Brisson in 1760, with theanhinga or American darter (Anhinga anhinga) as thetype species.[13][14]Anhinga is derived from theTupiajíŋa (alsotranscribedáyinga orayingá), which in localmythology refers to a malevolentdemonic forest spirit; it is often translated as "devil bird". The name changed toanhingá oranhangá as it was transferred to the Tupi–PortugueseLíngua Geral. However, in its first documented use as an English term in 1818, it referred to anOld World darter. Ever since, it has also been used for the modern genusAnhinga as a whole.[15]
This family is very closely related to the otherfamilies in thesuborderSulae, i.e. thePhalacrocoracidae (cormorants and shags) and theSulidae (gannets and boobies). Cormorants and darters are extremely similar as regards their body and legskeletons and may besister taxa. In fact, several darterfossils were initially believed to be cormorants or shags (see below). Some earlier authors included the darters in the Phalacrocoracidae assubfamily Anhingina, but this is nowadays generally consideredoverlumping. However, as this agrees quite well with the fossil evidence,[16] some unite the Anhingidae and Phalacrocoracidae in asuperfamilyPhalacrocoracoidea.[17]
The Sulae are also united by their characteristicdisplay behavior, which agrees with thephylogeny as laid out byanatomical andDNA sequence data. While the darters' lack of many display behaviors is shared with gannets (and that of a few with cormorants), these are allsymplesiomorphies that are absent infrigatebirds,tropicbirds andpelicans also. Like cormorants but unlike other birds, darters use theirhyoid bone to stretch thegular sac in display. Whether the pointing display of mates is anothersynapomorphy of darters and cormorants that was dropped again in some of the latter, or whether it evolved independently in darters and those cormorants that do it, is not clear. The male raised-wing display seems to be a synapomorphy of the Sulae; like almost all cormorants and shags but unlike almost all gannets and boobies, darters keep theirwrists bent as they lift the wings in display, but their alternating wing-waving, which they also show before take-off, is unique. That they often balance with their outstretched wings during walking is probably anautapomorphy of darters, necessitated by their being plumper than the other Sulae.[18]
The Sulae were traditionally included in thePelecaniformes, then aparaphyletic group of "higher waterbirds". The supposed traits uniting them, like all-webbed toes and a bare gular sac, are now known to beconvergent, and pelicans are apparently closer relatives ofstorks than of the Sulae. Hence, the Sulae and the frigatebirds – and someprehistoric relatives – are increasingly separated as theSuliformes, which is sometimes dubbed "Phalacrocoraciformes".[19]
There are four living species of darters recognized, all in thegenusAnhinga,[20] although the Old World ones were often lumped together as subspecies ofA. melanogaster. They may form asuperspecies with regard to the more distinct anhinga:[21]
| Common name | Scientific name and subspecies | Range | Size and ecology | IUCN status and estimated population |
|---|---|---|---|---|
| Anhinga or American darter
| Anhinga anhinga (Linnaeus, 1766) Two subspecies
| southern United States, Mexico, Cuba, and Grenada, Brazil. | Size: Habitat: Diet: | LC |
| Oriental darter
| Anhinga melanogaster Pennant, 1769 | tropical South Asia and Southeast Asia. | Size: Habitat: Diet: | NT |
| African darter
| Anhinga rufa (Daudin, 1802) Two subspecies
| sub-Saharan Africa and Iraq. | Size: Habitat: Diet: | LC |
| Australasian darter
| Anhinga novaehollandiae Gould, 1847 | Australia, Indonesia, and Papua New Guinea | Size: Habitat: Diet: | LC |
Extinct "darters" fromMauritius andAustralia known only from bones were described asAnhinga nana ("Mauritian darter") andAnhinga parva. But these are actually misidentified bones of thelong-tailed cormorant (Microcarbo/Phalacrocorax africanus) and thelittle pied cormorant (M./P. melanoleucos), respectively. In the former case, however, the remains are larger than those of the geographically closest extant population of long-tailed cormorants onMadagascar: they thus might belong to an extinct subspecies (Mauritian cormorant), which would have to be calledMicrocarbo africanus nanus (orPhalacrocorax a. nanus) – quite ironically, as theLatin termnanus means dwarf. TheLate PleistoceneAnhinga laticeps is not specifically distinct from the Australasian darter; it might have been a largepaleosubspecies of thelast ice age.[22]
Thefossil record of the Anhingidae is rather dense, but veryapomorphic already and appears to be lacking its base. The other families placed in thePhalacrocoraciformes sequentially appear throughout theEocene, the most distinct – frigatebirds – being known since almost 50 Ma (million years ago) and probably ofPaleocene origin. With fossil gannets being known since the mid-Eocene (c. 40 Ma) and fossil cormorants appearing soon thereafter, the origin of the darters as a distinct lineage was presumably around 50–40 Ma, maybe a bit earlier.[23]
Fossil Anhingidae are known since theEarly Miocene; a number ofprehistoric darters similar to those still alive have been described, as well as some more distinct genera nowextinct. The diversity was highest inSouth America, and thus it is likely that the family originated there. Some of the genera which ultimately became extinct were very large, and a tendency to becomeflightless has been noted in prehistoric darters. Their distinctness has been doubted, but this was due to the supposed"Anhinga" fraileyi being rather similar toMacranhinga, rather than due to them resembling the living species:[24]
Prehistoric members ofAnhinga were presumably distributed in similarclimates as today, ranging intoEurope in the hotter and wetterMiocene. With their considerable stamina and continent-wide distribution abilities (as evidenced by the anhinga and the Old Worldsuperspecies), the smaller lineage has survived for over 20 Ma. As evidenced by the fossil species'biogeography centered around theequator, with the younger species ranging eastwards out of the Americas, theHadley cell seems to have been the main driver of the genus' success and survival:[26]
Protoplotus, a smallPaleogene phalacrocoraciform fromSumatra, was in old times considered a primitive darter. However, it is also placed in its own family (Protoplotidae) and might be abasal member of the Sulae and/or close to the common ancestor of cormorants and darters.[37]
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