The white-shouldered ibis (Pseudibis davisoni) is a large species of bird in the family Threskiornithidae. It is native to small regions of Southeast Asia, and is considered to be one of the most threatened bird species on this part of the continent.
Taxonomy and systematics 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.
Description 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. 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. 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. 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.
Distribution and 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. 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. 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. 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.
Ecology
Feeding
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