The white-eyed river martin (Pseudochelidon sirintarae) is a passerine bird, one of only two members of the river martin subfamily of the swallows. Since it has significant differences from its closest relative, the African river martin, it is sometimes placed in its own genus, Eurochelidon. First found in 1968, it is known only from a single wintering site in Thailand, and may be extinct, since it has not been seen since 1980 despite targeted surveys in Thailand and neighbouring Cambodia. It may possibly still breed in China or Southeast Asia, but a Chinese painting initially thought to depict this species was later reassessed as showing pratincoles. The adult white-eyed river martin is a medium-sized swallow, with mainly glossy greenish-black plumage, a white rump, and a tail which has two elongated slender central tail feathers, each widening to a racket-shape at the tip. It has a white eye ring and a broad, bright greenish-yellow bill. The sexes are similar in appearance, but the juvenile lacks the tail ornaments and is generally browner than the adult. Little is known of the behaviour or breeding habitat of this martin, although like other swallows it feeds on insects caught in flight, and its wide bill suggests that it may take relatively large species. It roosts in reed beds in winter, and may nest in river sandbanks, probably in April or May before the summer rains. It may have been overlooked prior to its discovery because it tended to feed at dawn or dusk rather than during the day. The martin's apparent demise may have been hastened by trapping, loss of habitat and the construction of dams. The winter swallow roosts at the only known location of this martin have greatly reduced in numbers, and birds breeding at river habitats have declined throughout the region. The white-eyed river martin is one of only two birds endemic to Thailand, and the country's government has noted this through the issues of a stamp and a high-value commemorative coin.
Taxonomy Within the swallow family, the white-eyed river martin is one of only two members of the river martin subfamily Pseudochelidoninae, the other being the African river martin Pseudochelidon eurystomina of the Congo Basin in Africa. These two species possess a number of distinctive features which mark them out from other swallows and martins, including their robust legs and feet, and stout bills. The extent of their differences from other swallows and the wide geographical separation of the two martins suggest that they are relict populations of a group of species that diverged from the main swallow lineage early in its evolution. The separation of this subfamily is supported by genetic evidence, and their habit of nesting in burrows is thought to be characteristic of the earliest members of the swallow family. The white-eyed river martin was discovered in 1968 by Thai ornithologist Kitti Thonglongya, who gave the bird its current binomial name. The genus name Pseudochelidon (Hartlaub, 1861) comes from the Ancient Greek prefix ψευδο/pseudo "false" and χελιδον/chelidôn, "swallow", and the species name sirintarae commemorates Princess Maha Chakri Sirindhorn of Thailand. The African and Asian Pseudochelidon species differ markedly in the size of their bills and eyes, suggesting that they have different feeding ecologies, with the white-eyed river martin probably able to take much larger prey. The Thai species also has a swollen, hard gape (fleshy interior of the bill) unlike the softer, fleshier, and much less prominent gape of the African river martin. Thonglongya estimated the bill of the Thai species to be 17.6% wider than that of the African bird, but a later estimate, using specimens preserved in alcohol instead of dried skins (to avoid shrinkage), gave a difference of 22.5% between the bills of the two swallows. Following a suggestion by Kitti in his original paper, Richard Brooke proposed in 1972 that the white-eyed river martin was sufficiently different from the African species to be placed in a separate monotypic genus Eurochelidon, but this was contested by other authorities. The new genus was not subsequently widely adopted by other authors, although BirdLife International uses Eurochelidon.
Description
The adult white-eyed river martin is a medium-sized swallow, 18 cm (7 in) long, with mainly silky black plumage and a white rump. The back is green-glossed black, and is separated from the similarly coloured upper tail by a narrow bright white rump band. The head is darker than the back, with a velvet-black chin leading to blue-green glossed black underparts. The wings are black, with brown inner edges to the flight feathers, and the tail is green-glossed black with two elongated, slender, central tail feathers, up to 9 cm (3.5 in) long. These expand slightly at the tips to give narrow racquets 4.9–8.5 cm (1.9–3.3 in) long. The wing length averages 11.5 cm (4.5 in), the tail is 10.7 cm (4.2 in) long, and the tarsus averages 1.1 cm (0.43 in). The iris and eyelid are white, giving the appearance of a white eye ring, and the broad, bright greenish-yellow bill has a black hooked tip to the upper mandible. The large, strong feet and legs are flesh-coloured. This species is silent when wintering, and its breeding vocalisations are unknown. The sexes are similar, but the juvenile lacks the tail racquets, has a brown head and chin, and is generally browner than the adult. Juveniles taken in January and February were moulting their body feathers. The original Thai name for the Pseudochelidon, only known to local people in Bueng Boraphet, was นกตาพอง (nok ta phong) which may be roughly translated as "bird with enlarged eyes". After its official discovery in 1968, it was named นกเจ้าฟ้าหญิงสิรินธร (nok chaofa ying sirinthon, lit. "Princess Sirindhorn's bird").
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