Ibis spallabianca

Identificazione

The adult stands 60–85 cm tall, with males being slightly larger and having slightly longer bills than females. Its only available biometrics are measurements from a single unsexed specimen originating from the 19th century, which include a wing length of 419mm, culmen length of 197mm, tarsus length of 83mm and tail length of 229mm. The plumage is brownish-black, with glossy blue-black wings and tail, and a bare slate-black head which has also been reported as blue or white. A conspicuous neck collar comprising a bluish-white band of bare skin which is broader at the back and narrower at the front extends from the chin around to the nape at the base of the skull. The pale blue is most easily detectable at close range, although this collar may sometimes be completely white. The legs are dull red, the iris is orange-red, and the large de-curved bill is yellowish-grey.<p>The white-shouldered ibis owes its name to the clear white patch visible on the upper part of the neck and chin in some individuals. These white patches may appear as “white shoulders” in flight. In flight, it is also identified by its conspicuous white wing patch, which is visible only as a thin white line when the wings are closed. The white-shouldered ibis is morphologically similar to its Indian congener the black or red-naped ibis Pseudibis papillosa, but lacks the red tubercles on the nape; and is slightly larger, more robust and has a longer neck and legs. The tail also appears shorter and spreads downwards, not straight as in the black ibis.<p>The juvenile has dull-brown plumage with a tuft of brown feathers on the bluish-white nape, a grey-brown iris, pale yellow legs and dull white feet.<p>Vocalizations generally consist of loud, mournful calls that have been described as “weird and unearthly screams”. The hoarse calls of territorial individuals have been described as “errrrh” or “errrrrroh”. It also utters honking screams of “errrrh owk owk owk owk owk” and more subdued “ohhaaa ohhaaa” and “errrrrah”. It makes a loud, harsh “klioh klioh” call during copulation, resembling that of the black woodpecker.

Tassonomia

The white-shouldered ibis was first described by Hume (1875), who originally named the species Geronticus davisoni after his bird collector William Ruxton Davison. Based on this species’ observed similarity with the black ibis (Elliot, 1877), the two species were placed in the same genus. In the more recent past, this ibis has often been classified as a subspecies of the black ibis; but is currently recognised as a separate species.

Nome scientifico

Pseudibis davisoni

Autorità

(Hume, 1875)

Ordine

Pelecaniformi

Famiglia

Threskiornithidae

Genere

Pseudibis

Areale e habitat

This southeast Asian ibis was once much more widespread than presently. It used to extend throughout Southeast Asia from Myanmar to Thailand, Malaysia, Cambodia, Laos, Vietnam and north into Yuman in China. The current population is very small and its distribution highly fragmented, being restricted to northern and eastern Cambodia, southern Vietnam, extreme southern Laos and East Kalimantan.<p>Cambodia, the species' largest stronghold, holds an estimated 85-95% of the global population. The largest known Cambodian white-shouldered ibis subpopulation resides in Western Siem Pang Important Bird Area (minimally 346 individuals). Other sites in Cambodia that hold considerable numbers of white-shouldered ibis include Kulen Promtep Wildlife Sanctuary, Lomphat Wildlife Sanctuary and the central section of the Mekong River. It is now functionally extinct from Thailand, Myanmar, and southern China; and very scarce in Indonesian Borneo and southern Laos. Thailand was once the stronghold for this species, but no official records of its occurrence have been made here since 1937.<p>The white-shouldered ibis is a lowland specialist and has been observed in various habitats including dry dipterocarp forest, margins of seasonal pools (these pools are known locally as “trapaengs”) interspersed within forest, fallow rice fields, shrubby grasslands, forested lakesides and river margins, gravel and shingle banks at low river levels, sandbanks at wide rivers and, on the Sekong river, sandy islands. At least in Indochina, dry dipterocarp forest seems to be the most important habitat. However, one field study of the local population around the Mekong River in Cambodia found that the ibises nested both in flooded riverine forest and dry inland dipterocarp forest; which was a combination of utilised habitats not observed for any other population.<p>This species appears to capitalize on traditional local agricultural practices that create and maintain its preferred microhabitats, specifically through grazing and trampling of forest vegetation by livestock such as domestic cattle and water buffalo to create clearings as space for preferential foraging habitat; and through wallowing of ungulates in mud to create seasonal pools. This ibis's reliance on human-mediated activity may be especially strong considering both the marked local population declines of many wild ungulates in the white-shouldered ibis's range in the past few decades and the local extinction of many other species such as the Asian elephant; although wild boar may still be important contributors to creating seasonal pools through wallowing. These anthropogenic processes of creating and nurturing suitable habitats may be especially important in the early dry season when such habitat conditions are limited. Anthropogenic burning practices in dipterocarp forest may also have a similar role to grazing for creating suitable clearings.

Migrazione

Resident species

Comportamento

Unlike other sympatric wading birds, the white-shouldered ibis is largely terrestrial in its foraging strategy and has not been seen foraging in open water. It preferentially forages in muddy seasonal pools covered with short vegetation less than 25 cm high, on the ground in dry dipterocarp forest preferentially with bare underlying substrate, in fallow rice fields, and more occasionally in river channels with large extents of mud and sand. However, it forages almost exclusively at seasonal pools during the breeding season, probably due to high densities of refuge-seeking prey residing between the frequent cracks in the dry mud and hence high availability of food to feed to chicks. With its highly maneuverable, penetrative downcurved bill, the white-shouldered ibis is well adapted to probing into the cracks potentially harbouring concealed prey. This specific bill morphology gives the ibis an adaptive feeding advantage over other sympatric waders with straight bills, and is therefore commonly followed and by kleptoparasitic species such as cattle egrets and Chinese pond herons. Individuals feed singly, in pairs, or in family groups (flocks of up to 14 being reported). Flock size is much larger in the wet (non-breeding) than the dry (non-breeding) season.<p>Its diet comprises small invertebrates such as large worms, mole crickets, leeches, insect and beetle larvae; amphibians such as Paddy Frogs Fejervarya limnocharis and frogs of Microhyla species; and eels. Although amphibians appear to form the bulk of the diet, the main prey taken at a given place or time may depend on the texture of the underlying substrate For example, intake rate of invertebrate prey has been found to be significantly higher in saturated than moist or dry grounds; and amphibian intake rate significantly greater in dry than moist, and moist than saturated grounds. However, dry substrate appears to be most favourable, probably because of the high level of available biomass constituted by large amphibians (Wright et al. 2013b). Incidentally, there are also unsubstantiated claims of this ibis feeding on fruit.<p>After rainfall during the wet season, individuals increasingly use forests and fields as foraging ground instead of pools, probably because of dilution of prey density in the water at pool margins. Additionally, swamp eels and crabs which are primarily aquatic and occur in saturated substrate have not been identified in white shouldered-ibis diet because these potential prey could easily evade predation by burrowing or swimming away.<p>Breeding pairs are demanding in their food requirements, with each pair in Cambodia estimated to utilise nearly two thirds of the total amphibian biomass at a given time for a water hole over an entire breeding season. Therefore, breeding pairs would need to use multiple waterholes for adequate sustenance over the breeding season to avoid prey depletion at one pool, and the supposedly high resultant intraspecific competition would lead to large dispersion of the breeding population. Therefore, any conservation recommendations for this species would need to consider a landscape scale approach.<p>The white-shouldered ibis is a solitary breeder. In its Cambodian stronghold, it nests December–April during the mid- to late dry season November – May in dipterocarp tree canopies. This breeding strategy contrasts with those in other sympatric water bird species which nest during the wet season or from the late wet to mid- dry seasons. However, the dry-season breeding in Cambodia is probably synchronized with water drawdown in seasonal pools because of high density of amphibian prey taking refuge in the cracks in the dry mud at pool margins; thereby increasing the potential food availability for chicks and breeding adults. Different breeding seasons have been reported elsewhere in its range; with February–March in Myanmar when it still persisted here, September–December in East Kalimantan, and late August–December in Borneo; with copulation observed in February in this region.<p>It builds large nests 10-25m aboveground, composed of twigs and fresh leaves, usually in emergent trees such as Koompasia excels as in Borneo. Both partners continue to add nesting material throughout the breeding season and the same nests may be used in successive years. It also more occasionally uses abandoned raptor nests. The clutch contains 2-4 eggs, which are pale blue and hatch asynchronously. The eggs are incubated for 28–31 days by the female. Available reported egg measurements are 61.0-68.2mm in length and 43.9-46.7mm in width. The nestling period from hatching to fledging lasts 26–40 days.<p>In contrast to their solitary breeding during the dry season, white-shouldered ibises become gregarious in the wet season July–October (non-breeding season); when they roost communally in trees. During the wet season, large congregations of white-shouldered ibis (up to 185 individuals at a time have been noted) have also been observed foraging on open terrestrial habitats such as abandoned paddy fields and less frequently at seasonal pools with higher water levels than during the dry season (during breeding).<p>The white-shouldered ibis is considered sedentary, but some small movements of just over 5 km between roosting and foraging grounds may occur during the wet season. During the wet season in Cambodia, there is considerable movement from pools and wet ground in the dipterocarp forest towards drier forest ground, probably due to easy accessibility of terrestrial invertebrate prey compared to evasive amphibians at the pools. On Borneo, white-shouldered ibises move along large rivers such as the Mahakam in response to large fluctuations in water levels and hence spatiotemporal variations of exposed river banks that serve as suitable feeding grounds. Additionally, large-scale forest fires in East Kalimantan induced by the El Nino Southern Oscillation in the mid-1990s caused large-scale habitat destruction, leading to appreciable movements of individuals into unburnt forest stretches and thereby resulting in a more aggregated local population distribution.

Conservazione

Tendenza della popolazione

declining