Anhingidaeanhingas and darters

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Anhingidae comprises a single genus (Anhinga). There remains disagreement concerning the number of species within Anhingidae, but four species are generally recognized.

Anhingids are distributed throughout tropical, subtropical and warm temperate zones.

Anhingids mainly inhabit fresh water lakes, rivers, marshes and swamps. But they are also found in marine or brackish waters such as estuaries, bays, lagoons and mangrove swamps.

Eggs are incubated for 25-30 days and hatch asynchronously. Chicks are altricial and acquire a white or tan down within two days. Chicks are fed and brooded by parents. Fledging occurs at about five weeks and post-fledging feed continues for approximately two weeks. Sexual maturity is achieved at about two years. Anhingids may survive for nine years in the wild and 16 years in captivity.

Anhingas and darters are large birds (81-97 cm; 1058-1350 g; 116-128 cm wingspan), with extremely long, slender necks, and turkey-like tails with stiff retrices. The sexes are monomorphic in size, but males generally have larger bills. Plumage is dimorphic: males are black to dark brown whereas females are paler, particularly on neck and underparts. Males have an erectile crest on the nape. Both sexes have extensive gray stippling on long scapulars and upper wing coverts. Juvenile plumage is browner than the female's and lacks stippling. Seasonally variable iris color may be yellow, red, or brown. During breeding the small bare gular sac changes from pink or yellow to black, and bare facial skin changes from yellow or greenish-yellow to turquoise. The bill is sharply pointed with serrated edges, desmognathous palate and nostrils obsolete (no exposed external nares). Short legs are set far back on body, with totipalmate feet. Ventral keel present on cervical vertebrae 5-7 provides attachment for muscles that project bill forward like a spear.

Some anhingids breed in colonies alongside cormorants or herons. Or can be seen flying with ibises, herons, or storks.

Anhingids feed primarily on fish (Cichildae, Centrarchidae, Poeciliidae, Cyprinodontidae, Cyprinidae, and Plotosidae. Mugilidae). They will also prey on: amphibians (salamanders, frogs, newts), reptiles (snakes, baby alligators, turtles), and invertebrates (insects, leeches, mollusks, crayfish, shrimp).

Anhingid predators include: crocodile (Crocodilis), raven (Corvus coronoides), marsh harrier (Circus aeruginosus), House Crow (Corvus splendens), Pallas' Eagle (Haliaeetus leucorphus).

The duration of pair bond is not known, but anhingids appear minimally to be seasonally monogamous. Mating displays are diverse and vary with species. Males may advertise using wing waving, snap bowing, or twig grasping. Greeting displays include twig shaking, snap bowing, gaping or wing lifting. Pair bonding displays may include bill rubbing, sky-pointing, simultaneous neck waving or bowing. Copulation occurs at the nest site.

Anhingas and darters breed in colonies. Breeding is either seasonal (peaking in March or April) or year round, and varies with geographic range. Nests constructed of twigs are located in trees or reeds, and frequently located near or above water. Generally, the male brings twigs to the nest site, while the female places the twigs. The nest is built in one to three days. Clutch size ranges from two to six eggs. Egg laying interval is one to two days. Eggs are pale green.

Male and female take turns incubating eggs (using foot webbing) continuously for 25-30 days. Both parents brood and feed chicks for about five weeks. Young chicks are fed partially digested prey items, whereas older chicks are fed solid food items taken from the parents' throat. Parents continue to feed chicks for about two weeks after chicks have fledged.

Anhingids are generally sedentary, although populations at extremes of the distribution may migrate. With large, broad wings, anhingids are excellent aerial soarers and gliders. Anhingids often swim with the body underwater with the neck and head above water. (The neck often takes on an S-shape or snake-like configuration, from whence the term snakebird is derived). From this posture, the birds quietly submerge to stalk or ambush prey items underwater. Anhingids foot-propel themselves slowly underwater, then suddenly thrust the bill forward to spear fish in the flank. (A hinge mechanism in the cervical vertebrae enables anhingids to quickly snap the head forward rapidly to spear prey items). Speared fish are brought to the surface where anhingids toss the fish into the air, catch the fish in the bill, and swallow the fish headfirst.

Anhingids may gather in flocks of up to about 100 birds. Anhingids are highly territorial and agonistic interactions are common amongst males.

Specific and distinguishable vocalizations characterize the different sexes, ages and species. Anhingids produce clicking or rattling when flying or perching. At the nest adults use croaks, grunts, or rattles, whereas chicks may squeal or squawk. Breeding adults may emit caws, sighs or hissing calls.

Anhingid eggs and birds are collected for human consumption. Humans use anhingids as well as cormorants in conjunction with fishing. (see Phalacrocoracidae)

One species, Oriental Darter (Anhinga melanogaster), is listed as Lower Risk in the IUCN Red List of Threatened Species. Threats include habitat destruction, collection of birds and eggs, and pesticide poisoning.

The evolutionary relationships of anhingas and darters remain unclear. Anhingids are generally considered closely related to other totipalmate birds (tropicbirds, frigatebirds, cormorants and shags, sulids, pelicans), which when taken together, form Pelecaniformes.

An hierarchy based on DNA hybridization includes anhingids within a diverse group, Ciconiiformes. Some evidence suggests Phalacrocoracidea comprises anhingids along with cormorants and shags, while other evidence supports anhingids alone within Anhingidae and sister to Phalacrocoracidae. The number of anhingid species recognized remains inconsistent, with one, two or four species acknowledged. Morphological, ethological and molecular analyses suggest several hypotheses of sister group relationships: anhingids as sister to cormorants and shags forming a group sister to sulids; anhingids as sister to sulids forming a group sister to cormorants and shags; anhingids as sister to a group comprising cormorants and shags and sulids.

Old and New World anhingid fossils have been dated from the early (Anhinga subvolans) and the late (A. pannonica, A. grandis) Miocene. A. anhinga fossil remains from Florida date to the Pleistocene.

Campbell, B., and E. Lack, editors. 1985. A Dictionary of Birds. Buteo Books, Vermillion, SD.

del Hoyo, J., Elliott, A. & Sargatal, J. (eds.) 1992. Handbood of the Birds of the World. Vol. 1. Lynx Edicions, Barcelona.

Feduccia, A. 1999. The Origin and Evolution of Birds, 2nd edition. Yale University Press New Haven.

Johnsgard, P. A. 1993. Cormorants, Darters, and Pelicans of the World. Smithsonian Institution Press Washington.

Sibley, C. G. & J. E. Ahlquist. 1990. Phylogeny and Classification of Birds, A Study in Molecular Evolution. Yale Univ. Press.

Contributors

Laura Howard (author), Animal Diversity Web.

Glossary

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

visual

uses sight to communicate