Twin rails travel apart

Water Rail has now been shown to consist of two sister species.

Already split in Mark Brazil's Birds of East Asia, among others, the separation of the eastern form of Water Rail has now received genetic support.

A team from the Royal Ontario Museum, Canada, sampled museum specimens collected from across the range of this widespread Palearctic form, confirming the existence of three Water Rail Rallus aquaticus subspecies, but also reaffirming that eastern birds from China, North and South Korea and Japan form their own strongly divergent species, R indicus, which they have named Brown-cheeked Rail. The three per cent divergence was noticeable in all three genes used for the analysis, and the form is believed to have split at least 534,000 years ago, after ancestral Water Rails spread over the whole Palearctic region about two million years ago.

Water Rail itself is distributed from western China across Eurasia and into Iceland, which has its own insular subspecies R aquaticus hibernans. The central Asian population is isolated and distinguishable as R a korejewi, but all of Europe's birds bar Iceland's are nominate, as there appears to be almost constant gene flow between populations. The separation of the three continental populations in Eurasia is believed to have been the result of wetlands becoming more disjunct as glaciation progressed during the Ice Age.

Apart from on range, Brown-cheeked Rail ('Eastern Water Rail' in Brazil 2009) is identifiable by call, which incorporates some piping notes not found in R aquaticus, and its face pattern – with a white supraloral stripe and a brown eyestripe – and brown-washed breast coloration.

Reference: Tavares, E, de Kroon, G H J and Baker, A J. 2010. Phylogenetic and coalescent analysis of three loci suggest that the Water Rail is divisible into two species, Rallus aquaticus and R. indicus. BMC Evol Biol 10: 226.