Greater adjutant

Description

The greater adjutant is a huge bird, standing tall at 145–150 cm (57–59 in). The average length is 136 cm (54 in) and average wingspan is 250 cm (98.5 in). While no weights have been published for wild birds, the greater adjutant is among the largest of living storks, with published measurements overlapping with those of the jabiru (Jabiru mycteria), saddle-billed stork (Ephippiorhynchus senegalensis) and marabou stork (Leptoptilos crumeniferus). Juvenile greater adjutant storks in captivity weighed from 8 to 11 kg (18 to 24 lb). For comparison, the heaviest known wild stork was a marabou stork scaling 8.9 kg (20 lb). The huge bill, which averages 32.2 cm (12.7 in) long, is wedge-like and is pale grey with a darker base. The wing chord averages 80.5 cm (31.7 in), the tail 31.8 cm (12.5 in) and the tarsus 32.4 cm (12.8 in) in length. With the exception of the tarsus length, the standard measurements of the greater adjutant are on average greater than that of other stork species. A white collar ruff at the base of its bare yellow to red-skinned neck gives it a vulture-like appearance. In the breeding season, the pouch and neck become bright orange and the upper thighs of the grey legs turn reddish. Adults have a dark wing that contrasts with light grey secondary coverts. The underside of the body is whitish and the sexes are indistinguishable in the field. Juveniles are a duller version of the adult. The pendant inflatable pouch connects to the air passages and is not connected to the digestive tract. The exact function is unknown, but it is not involved in food storage as was sometimes believed. This was established in 1825 by Dr. John Adam, a student of Professor Robert Jameson, who dissected a specimen and found the two-layered pouch filled mainly with air. The only possible confusable species in the region is the smaller lesser adjutant (Leptoptilos javanicus), which lacks a pouch, prefers wetland habitats, has a lighter grey skull cap, a straighter edge to the upper mandible, and lacks the contrast between the grey secondary coverts and the dark wings.

Like others storks, it lacks intrinsic muscles in the syrinx and produces sound mainly by bill-clattering, although low grunting, mooing or roaring sounds are made especially when nesting. The bill-clattering display is made with the bill raised high and differs from that of the closely related African marabou which holds the bill pointed downwards.

Taxonomy and systematics

John Latham wrote about the bird found in Calcutta based on descriptions in Ives's Voyage to India published in 1773 and included an illustration in the first supplement to his General Synopsis of Birds. The illustration was based on a drawing in Lady Impey's collection. Latham called it the "gigantic crane", and included observations by an African traveller named Smeathman who described a similar bird from western Africa. Johann Friedrich Gmelin used Latham's description and described the Indian bird as Ardea dubia in 1789 while Latham later used the name Ardea argala for the Indian bird. Temminck used the name Ciconia marabou in 1824 based on the local name used in Senegal for the African bird and this was also applied to the Indian species. This led to considerable confusion between the African and Indian species. The marabou stork of Africa looks somewhat similar but their disjunct distribution ranges, differences in bill structure, plumage, and display behaviour support their treatment as separate species.

Most storks fly with their neck outstretched, but the three Leptoptilos species retract their neck in flight as herons do, possibly due to the heavy bill. When walking on the ground, it has a stiff marching gait from which the name "adjutant" is derived.

Distribution

This species was once a widespread winter visitor in the riverine plains of northern India. However, their breeding areas were largely unknown for a long time until a very large nesting colony was finally discovered in 1877 at Shwaygheen on the Sittaung River, Pegu, Burma. It was believed that the Indian birds bred there. This breeding colony, which also included spot-billed pelicans (Pelecanus philippensis), declined in size and entirely vanished by the 1930s. Subsequently, a nest site in Kaziranga was the only known breeding area until new sites were discovered in Assam, the Tonle Sap lake and in the Kulen Promtep Wildlife Sanctuary. In 1989, the breeding population in Assam was estimated at about 115 birds, and between 1994 and 1996 the population in the Brahmaputra valley was considered to be about 600. A small colony with about 35 nests was discovered near Bhagalpur in 2006. The number increased to 75 nests in 2014. Fossil evidence suggests that the species possibly (since there were several other species in the genus that are now extinct) occurred in northern Vietnam around 6000 years ago.

During the non-breeding season, storks in the Indian region disperse widely, mainly in the Gangetic Plains. Sightings from the Deccan region are rare. Records of flocks from further south, near Mahabalipuram, have been questioned. In the 1800s, adjutant storks were extremely common within the city of Calcutta during the summer and rainy season. These aggregations along the Ghats of Calcutta, however, declined and vanished altogether by the early 1900s. Improved sanitation has been suggested as a cause of their decline. Birds were recorded in Bangladesh in the 1850s, breeding somewhere in the Sundarbans, but have not been recorded subsequently.

Behaviour and ecology

The greater adjutant is usually seen singly or in small groups as it stalks about in shallow lakes or drying lake beds and garbage dumps. It is often found in the company of kites and vultures and will sometimes sit hunched still for long durations. They may also hold their wings outstretched, presumably to control their temperature. They soar on thermals using their large outstretched wings.

Breeding

The greater adjutant breeds during winter in colonies that may include other large waterbirds such as the spot-billed pelican. The nest is a large platform of twigs placed at the end of a near-horizontal branch of a tall tree. Nests are rarely placed in forks near the center of a tree, allowing the birds to fly easily from and to the nests. In the Nagaon nesting colony in Assam, tall Alstonia scholaris and Anthocephalus cadamba were favourite nest trees. The beginning of the breeding season is marked by several birds congregating and trying to occupy a tree. While crowding at these sites, male birds mark out their nesting territories, chasing away others and frequently pointing their bill upwards while clattering them. They may also arch their body and hold their wings half open and drooped. When a female perches nearby, the male plucks fresh twigs and places it before her. The male may also grasp the tarsus of the female with the bill or hold his bill close to her in a preening gesture. A female that has paired holds the bill and head to the breast of the male and the male locks her by holding his bill over her neck. Other displays include simultaneous bill raising and lowering by a pair. The clutch, usually of three or four white eggs, is laid at intervals of one or two days and incubation begins after the first egg is laid. Both parents incubate and the eggs hatch at intervals of one or two days, each taking about 35 days from the date of laying. Adults at the nest have their legs covered with their droppings and this behaviour termed as urohidrosis is believed to aid in cooling during hot weather. Adults may also spread out their wings and shade the chicks. The chicks are fed at the nest for about five months. The chicks double in size in a week and can stand and walk on the nest platform when they are a month old. At five weeks, the juveniles leap frequently and can defend themselves. The parent birds leave the young along for longer periods at nest at this stage. The young birds leave the nest and fly around the colony when about four months but continue to be fed occasionally by the parents.

Feeding

The greater adjutant is omnivorous and although mainly a scavenger, it preys on frogs and large insects and will also take birds, reptiles and rodents. It has been known to attack wild ducks within reach and swallowing them whole. Their main diet however is carrion, and like the vultures their bare head and neck is an adaptation. They are often found on garbage dumps and will feed on animal and human excreta. In 19th-century Calcutta, they fed on partly burnt human corpses disposed along the Ganges river. In Rajasthan, where it is extremely rare, it has been reported to feed on swarms of desert locusts (Schistocerca gregaria) but this has been questioned.

Parasites, diseases and mortality

At least two species of bird lice, Colpocephalum cooki and Ciconiphilus temporalis have been found as ectoparasites. Healthy adult birds have no natural predators, and the only recorded causes of premature mortality are due to the direct or indirect actions of humans; such as, poisoning, shooting, or electrocution such as when the birds accidentally fly into telephone lines. Captive birds have been found to be susceptible to avian influenza, (H5N1) and a high mortality rate was noted at a facility in Cambodia, with two-thirds of infected birds dying. The longest recorded life span in captivity was 43 years.

Status and conservation

Loss of nesting and feeding habitat through the draining of wetlands, pollution and other disturbances, together with hunting and egg collection in the past has caused a massive decline in the population of this species. The world population was estimated at less than 1,000 individuals in 2008. The greater adjutant is listed as Endangered on the IUCN Red List of Threatened Species.

Conservation measures have included attempts to breed them in captivity and to reduce fatalities to young at their natural nesting sites. Nearly 15% of the chicks are killed when they fall off the nests and die of starvation, so some conservationists have used nets positioned below the nests to prevent injuries to falling young and then raising these fallen birds in enclosures for about five months before releasing them to join their wild siblings.

In Kamrup district, Assam, which is home to one of the few large colonies of greater adjutants, outreach efforts including cultural and religious programming, especially aimed at village women, have rallied residents to conserve the birds. The locals, who formerly regarded the birds as pests, now see the storks as special and take pride in protecting them and the trees in which they breed. Locals have even added prayers for the safety of the storks to hymns, and included stork designs to the motifs used in traditional weaving. Similar measures have been used with success in other parts of India where adjutants breed.