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. but is currently recognised as a separate species. with males being slightly larger and having slightly longer bills than females. The plumage is brownish-black, with glossy blue-black wings and tail, and a bare slate-black head or white. 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 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. 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; on the ground in dry dipterocarp forest preferentially with bare underlying substrate, Both partners continue to add nesting material throughout the breeding season The nestling period from hatching to fledging lasts 26–40 days.
Threats and survival
thumb|right|Aspects of the head, by Arnoul in G. Masson (1878)
The white-shouldered ibis is considered one of the most threatened birds of SE Asia.
The majority of roosting individuals censused in Cambodia (where the largest known populations occur) during the wet season (about three quarters) have been found to occur outside protected areas, revealing an unfortunate spatial mismatch between important roosting sites and these protected areas.
Status
Populations of the white-shouldered ibis declined severely in the late 20th century. Given the scarcity of recorded sightings in the past few decades, the small population size and persistent habitat loss, it has been classified as Critically Endangered by the IUCN.