Mekong Wagtail: the great river's only known avian endemic
by Pete Davidson, Will Duckworth and Colin Poole, from OBC Bulletin 34, December 2001.
A new species of black-and-white wagtail has recently been described from the lower Mekong catchment of north-east Cambodia, southern Laos and, marginally, north-east Thailand.(1) It has been named Mekong Wagtail Motacilla samveasnae, its scientific binomial honouring the late Sam Veasna (pronounced 'Sam Veeshna'), one of Cambodia's leading ornithologists and conservationists, who tragically died of malaria in December 1999.(2) It is the only wagtail that breeds in the lower Mekong catchment, to which on current knowledge it is restricted.
Morphologically, Mekong Wagtail most closely resembles the widely disjunct African Pied Wagtail M. aguimp, though it differs in several minor respects, particularly wing pattern, and vocally it is highly distinct.(1) Moreover, mitochondrial DNA comparisons show that divergence between Mekong Wagtail and African Pied Wagtail is greater than between any of the other black-and-white wagtail taxa, and it also exhibits clear differences in moult strategy and habitat choice (1). It is highly distinctive, being the only black-and-white wagtail in South-East Asia exhibiting the adult/first-adult plumage character combination of a black forehead, lores and ear coverts, striking white supercilia, a white throat and white neck patch.(1,3)
Habitat specificity and parallels with other species
Mekong Wagtail is restricted to specific habitats of wide lowland river channels. Breeding birds are strongly associated with fast-flowing braided sections that bisect a distinctive mosaic of rocks, bushes adapted to prolonged seasonal submersion (predominantly Homonoia riparia), and some unconsolidated sediment (sandbars and gravel shoals)(1). These become inundated as river levels rise during the May/June-October/November rainy season. The few observations made to date during the high-flow season, all of which have been along stretches known to be used by breeding birds, indicate that the species concentrates along earthen banks and associated overhanging vegetation, and also restricted patches of exposed sand and silt, where they occur in pairs (some of which seem strongly territorial) and small flocks/feeding aggregations (of up to 12 individuals). It is unclear whether some birds make seasonal movements in response to rising water levels, but this is highly plausible given that some sections of channel mosaic and exposed riverbank used by breeding birds are completely submerged at the height of the high-flow season.
No other Eurasian wagtail exhibits such high habitat specificity. Although Grey Wagtail M. cinerea and Japanese Wagtail M. grandis are regarded as river/stream specialists, their habitat use is much broader than that of Mekong Wagtail.(3,4) White-browed Wagtail M. maderaspatensis and, in some areas, White Wagtails of the forms M. alba personata, M. alba alboides and M. alba leucopsis are mainly found at or near water, including along rivers, but none of them is nearly so specialised.(3,5,6) Non-breeding White Wagtails of the form M. alba leucopsis abound in southern Indochina during the palearctic winter, particularly along sandbars in rivers and earthen riverbanks, the very habitats generally shunned in the low-flow (breeding) season by Mekong Wagtails; conversely, in channel mosaic, leucopsis White Wagtail is less common, occurring mainly in sections with much sand. African Pied Wagtail uses a much wider variety of habitats,(1,7) including structurally similar channel mosaic habitat, at least in Gabon (JWD unpublished data).
The contrast in habitat use between Mekong and African Pied Wagtails parallels that of the closely related River Lapwing Vanellus duvaucelii and Spur-winged Plover V. spinosus (of south-east Europe, Africa and the Middle East): in both pairs, the Asian representative breeds only in river channels,8,9 while the African representative shows much wider habitat use, being common around non-flowing water.(10) Moreover, Indochinese and African populations of Wire-tailed Swallow Hirundo smithii show similar contrast in habitat use.(1,11)
Mekong Wagtail's strong association with channel mosaic, notably rocks and bushes, recalls that of Jerdon's Bushchat in the upper Lao Mekong.(12,13) The bushchat has not been found downstream of Vientiane, north Laos. Despite similarities in habitat occupied, size and generally (presumed) insectivorous diet, it is highly unlikely that these two are competitors: their feeding styles are entirely different. Some other factor, yet to be understood, must set their upstream and downstream distribution limits.
Threats to Mekong Wagtail and the wider river-channel bird community
The Mekong mainstream and the Kong, San and Srepok tributaries in Cambodia support healthy numbers of Mekong Wagtail, but the total area they occupy is small. Its linear distribution concentrates the population, making it highly susceptible to perturbations. The chief threat is dam construction, primarily for hydro-electric power generation, and associated changes in water and sediment flow patterns. Many dams have been proposed for the lower Mekong and its major tributaries (14) but for various reasons it is likely that only a small proportion will ever be built, and there is continual flux over which are most favoured. Additive effects on flow/dry-season channel flooding of even a small proportion of these proposed dams could well be very severe for the whole channel bird community, including Mekong Wagtail.
The species is far less susceptible to human activity factors (chiefly hunting, egg collection and disturbance) than are most other sympatric river-channel specialists, many of which are now close to regional extinction. Thai/Lao river channel bird populations have undergone severe declines,(15,16,17) and indeed, riverine biodiversity in its entirity in Indochina is in crisis.(18) Indian Skimmer Rynchops albicollis already appears to be regionally extinct. Black-bellied Tern Sterna acuticauda must now be perilously close to regional extinction, with just three records, perhaps involving just two pairs, over the past five years, despite extensive surveys.(19) A suite of other species have also suffered severe declines, including Great Thick-knee Esacus recurvirostris, River Lapwing, River Tern Sterna aurantia, Little Tern Sterna albifrons and Darter Anhinga melanogaster. Most of these are now restricted to the least disturbed stretches of channel mosaic habitat, particularly areas with extensive well-vegetated sandbars.
Lessons to be learned from the 'discovery'
The chronology of events leading this wagtail's 'discovery' provides us with some useful lessons in ornithological rigour. In summary, it transpires that the bird was in fact first collected back in December 1972, on a tributary of the Mun river, Ubon Ratchathani Province, north-east Thailand. These specimens, now held in the Thailand Institute of Scientific and Technological Research, Bangkok, (TISTR) were used to illustrate the form of White Wagtail M. a. alboides in Lekagul and Round's A Guide to the Birds of Thailand.(15) Wagtails roughly conforming to this illustration were found on numerous occasions during surveys in Laos between February 1993 and late 1999 (16,17,20,21) and in 1997 were suspected to breed in southern Laos (PD unpublished data). When security problems in Cambodia eased in the late 1990s, surveys also began to find this wagtail breeding widely along the Mekong and its larger tributaries in the north-east of the country.(19,22) M. a. alboides breeds no closer than northern Vietnam23 and north Laos.(16,20) Thus, these records would have represented a major extension of the form's known breeding range.
However, they in fact generated minimal interest because a) they apparently fitted a known taxon portrayed in an excellent field guide, and b) attention was focused on scarcer, declining river channel specialists, thus the relatively numerous wagtails seemed of no elevated conservation concern. As a consequence, sightings of black-and-white wagtails were frequently incompletely documented, and, most importantly (in hindsight), not ascribed to recognisable subspecies. This has resulted in our current lack of knowledge over the species's range limits, specifically its northern limit along the Mekong, how far up some of her larger tributaries the species occurs, and indeed how abundant it is in southern Laos. It was not until Robson's Field Guide to the Birds of South-East Asia (9) was published, which accurately illustrates M. a. alboides (as indeed does Grimmett et al.'s Birds of the Indian Subcontinent (6)), showing it to be quite distinct from the breeding wagtail of southern Indochina, that the penny finally dropped. Had museum collections been more comprehensively consulted from the outset (skin-checking was in fact thorough, but probably only covered a third or so of all species recorded), the bird's identity would likely have been determined some years ago.
Where to see Mekong Wagtail
Two of the most accessible areas to see the species are in Cambodia, one along the Mekong mainstream at Kampi, just north of Kratie, the other along the lower Kong and San rivers just upstream of Stung Treng.
Daily passenger boats depart Phnom Penh for Kratie (journey time five hours), and a half hour moto-taxi ride upriver from the town will bring you to Kampi, where numerous boats are available for hire. Kampi is also the most accessible (and best known) locality to see Irrawaddy Dolphin Orcaella brevirostris along the Mekong in Cambodia. Several pairs of wagtails inhabit the channel mosaic extending for 2-3 kms upstream of the deepwater pool favoured by the dolphins during the dry season.
At least three return flights a week operate between Phnom Penh and Stung Treng, where again it is easy to find a boat to hire and either take upstream a couple of kilometres, from where the wagtail is common - one of the best stretches of river is along San just upstream of its confluence with the Kong, or head downstream into the Mekong main channel and explore the mosaic habitat both up and downstream of the Mekong/Kong confluence.
Alternatively, the stretch of the Mekong main channel in extreme southern Laos between Khon Falls and the Cambodian border is another good place to look, this area also supporting several Irrawaddy Dolphins.
We extend our sincere gratitude to Per Alström, Rob Timmins and Tom Evans for technical assistance in describing the wagtail, Tan Setha for logistical expertise and his effervescent company during recent survey work in Cambodia. Grateful thanks also to the Royal Government of Cambodia's Department of Forestry and Wildlife, Ministry of Agriculture, Forestry and Fisheries, and the Lao PDR Government Department of Forestry's Centre for Protected Areas and Watershed Management for their invaluable assistance in all manner of areas during the course of recent surveys in the two countries, and in particular the Wildlife Conservation Society Lao and Cambodia programmes, under which much of the work has been carried out. See the Bulletin of the British Ornithologists Club (1) for comprehensive acknowledgements pertaining to the type description of the wagtail and all those contributing information/expertise.
Duckworth, J. W., Alström, P., Davidson, P., Evans, T.D., Poole, C.M., Tan Setha & Timmins, R.J. 2001. A new species of wagtail from the lower Mekong basin. Bull. British Ornithologists' Club 121 (3): 152-182.
Poole, C.M. 2000. Obituary: Sam Veasna. Bull. Oriental Bird Club 31: 8.
Alström, P., Mild, K. & Zetterström, B. In press. Pipits and Wagtails of Europe, Asia and North America. London: Christopher Helm/A&C Black.
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