White-eyed River Martin

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. …

The white-eyed river martin was discovered in 1968 by Kitti Thonglongya, who obtained nine specimens netted by professional bird-hunters as part of a migratory bird survey at a night-time roost at Thailand's largest freshwater lake, Bueng Boraphet, in Nakhon Sawan Province. It was first seen in the wild by ornithologists at the same wintering site in 1977. The species has only been seen at the lake, always between the months of November and February, and the wintering habitat is assumed to be in the vicinity of open fresh water for feeding, with reed beds for the night-time roost. The white-eyed river martin may be migratory, and if the breeding habitat resembles that of the African river martin, it is likely to be the forested valleys of large rivers; these can provide sandbars and islands for nesting, and woodland over which the birds can catch insect prey. The breeding grounds and habitat are unknown, although river valleys in northern Thailand or southwestern China are possibilities. A claimed depiction of this species in a Chinese scroll painting initially appeared to support the possibility of the martin breeding in China. The bird in the painting had a similarly shaped head and bill, a white eye and a long tail, although it lacked the white rump, did not show the correct bill colour, and elongated the outer, rather than central, tail feathers. …

Since its breeding grounds are undiscovered, nothing is known about the white-eyed river martin's breeding biology, although it is suggested that it may nest in burrows in river sandbars, probably in April or May before the monsoon rain raises water levels. However, distinct differences in foot and toe morphology from its African relative have led some authorities to speculate that even the assumption that it nests in burrows could be incorrect. In winter, it roosts with barn swallows in reed beds. Like other swallows, the white-eyed river martin feeds on insects, including beetles, which are caught on the wing. Given its size and unusual mouth structure, it may well take larger insects than other swallows. This species is described as graceful and buoyant in flight, and, like its African relative, appears reluctant to use perches, behaviour that, together with its unusual toe-shape and the fact that mud was found on the toes of one of the first specimens, suggest that this species may be relatively terrestrial. Pamela C. Rasmussen suggested that, given its unusually large eyes, the species might be nocturnal or at least crepuscular, a factor that could make it very inconspicuous, and thus partly explain how it remained undetected for so long. …