Mekong Wagtail: the great river's only known avian endemic: 1by Pete Davidson, Will Duckworth and Colin Poole, from OBC Bulletin 34, December 2001. Introduction 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 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. |
