White-shouldered Ibis

In contrast to their dry season solitary breeding, 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). 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.

This southeast Asian ibis was once markedly more widespread than presently. The former range extended 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 by far forms this species’ stronghold; with 85-95% of the global population of individuals estimated here. 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 (Birdlife International, 2001).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 found to occur 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 lake 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 also appears to largely rely on traditional local agriculture to create and maintain its favoured 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.

Populations of the white-shouldered ibis declined severely late in the 20th century; and 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. The global population is estimated at 1000 individuals, an estimated 670 of which are mature. 973 individuals were counted in a nationwide population census in Cambodia in 2013, whereas 30-100 individuals are estimated for the Indonesian portion of the species’ range.