The black-necked stork (Ephippiorhynchus asiaticus) is a tall, long-necked wading bird in the stork family. It is a resident species across the Indian subcontinent and Southeast Asia with a disjunct population in Australia. It lives in wetland habitats and near fields of certain crops such as rice and wheat where it forages for a wide range of animal prey. Adult birds of both sexes have a heavy bill and are patterned in white and irridescent blacks, but the sexes differ in the colour of the iris with females sporting yellow irises and males having dark-coloured irises. In Australia, it is known as a jabiru, although that name also refers to a stork species found in the Americas. It is one of the few storks that are strongly territorial when feeding and breeding.
Taxonomy and systematics
First described by John Latham as Mycteria asiatica, this species was later placed in the now-defunct genus Xenorhynchus based on morphology. Based on behavioural similarities, M. Philip Kahl suggested the placement of the species in the genus Ephippiorhynchus, which then included a single species, the saddle-billed stork. This placement of both the black-necked stork and saddle-billed stork in the same genus was later supported by osteological and behavioural data, and DNA-DNA hybridisation and cytochrome–b data. The genera Xenorhynchus and Ephippiorhynchus were both erected at the same time and, as first revisor, Kahl selected the latter as the valid genus for the two species. This and the saddle-billed stork (Ephippiorhynchus senegalensis) are the only stork species that show marked sexual dimorphism in iris colour.
Two subspecies are recognized: E. a. asiaticus of the Oriental region and E. a. australis of south New Guinea and Australia. Charles Lucien Bonaparte erected the genus Xenorhynchus in 1855 and placed two species in it, X. indica and X. australis. This treatment was carried on into later works. James Lee Peters in his 1931 work treated them as subspecies. In 1989, McAllan and Bruce again suggested the elevation of the two subspecies into two species: E. asiaticus or the green-necked stork of the Oriental region, and E. australis or the black-necked stork of the Australian and New Guinean region. This recommendation was based on the disjunct distributions and differences in the iridescent colouration of the neck which the authors suggested might reflect different behavioural displays. This recommendation has not been followed and a subsequent study did not find consistent differences in the colours. Analysis of the cytochrome b mitochondrial sequences, however, showed significant genetic divergence. The genetic distance of a stork presumed to be Ephippiorhynchus asiaticus asiaticus from a confirmed individual of E. a. australis was 2.1%, much greater than the genetic distances between individual storks of the same species. The conservative treatment as two subspecies has been followed in the Australian faunal list by Christidis and Boles as well as internationally in the Clements checklist.
Description
The black-necked stork is a large bird, tall having a wingspan. The only published weight for this species was a single specimen at , but this is nearly 35% less than the mean body mass of the closely related and similar sized saddle-billed stork. Therefore, this specimen of black-necked stork could have been at the low end of sizes attainable or perhaps somewhat malnourished. The plumage patterns are conspicuous with younger birds differing from adults. Adults have a glossy bluish-black iridescent head, neck, secondary flight feathers and tail; a coppery-brown crown; a bright white back and belly; bill black with a slightly concave upper edge; and bright red legs. The sexes are identical in all aspects except the iris; the adult female has a yellow iris while the adult male has it brown. Juveniles younger than six months have a brownish iris; a distinctly smaller and straighter beak; a fluffy appearance; brown head, neck, upper back, wings and tail; a white belly; and dark legs. Juveniles older than six months have a mottled appearance especially on the head and neck where the iridescence is partly developed; dark-brown outer primaries; white inner primaries that forms a shoulder patch when the wings are closed; a heavy beak identical in size to adults but still straighter; and dark to pale-pink legs. Like most storks, the black-necked stork flies with the neck outstretched, not retracted like a heron. In flight it appears spindly and a black bar running through the white wings (the somewhat similar looking migratory black stork has an all black wing). This, along with the black neck and tail, makes it distinctive.
Distribution and habitat
In India, the species is widespread in the west, central highlands, and northern Gangetic plains extending east into the Assam valley, but rare in peninsular India and Sri Lanka. This distinctive stork is an occasional straggler in southern and eastern Pakistan, and is a confirmed breeding species in central lowland Nepal. It extends into Southeast Asia, through New Guinea and into the northern half of Australia. Compared to other large waterbirds like cranes, spoonbills and other species of storks, black-necked storks are least abundant in locations that have a high diversity of large waterbird species.
The largest population of this species occurs in Australia, where it is found from the Ashburton River, near Onslow, Western Australia, across northern Australia to north-east New South Wales. It extends inland in the Kimberley area to south of Halls Creek; in the Northern Territory to Hooker Creek and Daly Waters; and in Queensland inland to the Boulia area and the New South Wales border, with some records as far south as the north-west plains of New South Wales, along the coast of Sydney and formerly bred near the Shoalhaven River. It is rare along the south-east extremity of its range, but common throughout the north. An estimated 1,800 occur in the Alligator Rivers region of the Northern Territory, with overall numbers during surveys being low in all seasons. A combination of aerial surveys and ground counts in the middle Fly River floodplain, Papua New Guinea estimated 317 storks as of December 1994 and 249 as of April 1995.
The largest known breeding population occurs in the largely agricultural landscape of south-western Uttar Pradesh in India. Densities of about 0.099 birds per square kilometre have been estimated in this region made up of a mosaic of cultivated fields and wetlands. About six pairs lived in the of Keoladeo National Park. One breeding pair has been observed in Bhagalpur district, western Bihar.
In Sri Lanka, the species is a rare breeding resident, with four to eight breeding pairs in Ruhuna National Park. It is exceedingly rare, and possibly no longer breeding in Bangladesh and Thailand.
Black-necked storks forage in a variety of natural and artificial wetland habitats. They frequently use freshwater, natural wetland habitats such as lakes, ponds, marshes, flooded grasslands, oxbow lakes, swamps, rivers and water meadows. Freshwater, artificial wetland habitats used by these storks include flooded fallow and paddy fields, wet wheat fields, irrigation storage ponds and canals, sewage ponds, and dry floodplains. Small numbers are also seen in Indian coastal wetland habitats, including in mangrove creeks and marshes. In cultivated areas, they prefer natural wetlands to forage in, though flooded rice paddies are preferentially used during the monsoon, likely due to excessive flooding of lakes and ponds. Bird nests are usually on trees located in secluded parts of large marshes or in cultivated fields as in India and lowland Nepal.
Behaviour and ecology This large stork has a dance-like display. A pair stalk up to each other face to face, extending their wings and fluttering the wing tips rapidly and advancing their heads until they meet. They then clatter their bills and walk away. The display lasts for a minute and may be repeated several times.
Nest building in India commences during the peak of the monsoon with most of the nests initiated during September to November, with few new nests built afterwards until January. They nest on large trees, sometimes isolated in large marshes, or in agricultural landscapes, on which they build a platform. On agricultural landscapes, human disturbance can cause nesting adults to abandon nests in some locations, but storks in other locations nest successfully. The nest is large, as much as 3 to 6 feet across and made up of sticks, branches and lined with rushes, water-plants and sometimes with a mud plaster on the edges. Nests may be reused year after year. The usual clutch is four eggs which are dull white in colour and broad oval in shape, but varies from one to five eggs. The exact incubation period is not known but is suspected to be about 30 days. The chicks hatch with white down which is replaced by a darker grey down on the neck within a week. The scapular feathers emerge first followed by the primaries.
Fledged young birds make a chack sound followed by a repeated wee-wee-wee call. Another call is a low high-pitched peeeeeu-peeeeu-peeeeu-peeu whistle of 10 to 12 notes with a ventriloquistic quality. Juveniles at nest also peep continuously to solicit food.
Adult birds take turns at the nest and when one returns to relieve the other, they perform a greeting display with open wings and an up and down movement of the head. Food is brought for the young chicks by the adults and regurgitated onto the nest platform. Adults stop feeding the young at the nest and begin to show aggression towards the chicks after they are about 3 or 4 months old. The young birds may stay on nearby for about a year but disperse soon. Typically one to three chicks fledge from successful nests, but up to five chicks fledge in years with high rainfall. The number of stork pairs that succeed in raising chicks, and the average size of fledged broods, are strongly related to monsoonal and post-monsoon rainfall, improving in years with more rainfall.
At the nest trees, which are typically tall with large boles and a wide canopy, the birds in Bharatpur competed with Indian white-backed vultures sometimes failing to nest due to the vultures. While many wetland birds are flushed by birds of prey, these storks are not usually intimidated and can be quite aggressive to other large water-birds such as herons and cranes. Adults aggressively defend small depressions of deep water against egrets and herons (at Malabanjbanjdju in Kakadu National Park, Australia), and drying wetland patches against waterbirds such as spoonbills and woolly necked storks at Dudhwa National Park, Uttar Pradesh, India.