The adult is 23–28 cm (9–11 in) long, and, like other rails, has a body that is flattened laterally, allowing it easier passage through the reed beds it inhabits.

The former subspecies R. indicus, has distinctive markings and a call that is very different from the pig-like squeal of the western races, and is now usually split as a separate species, the brown-cheeked rail.

The introduced American mink has exterminated some island populations, but overall the species' huge range and large numbers mean that it is not considered to be threatened.

Its Old World members, the water, African and Madagascar rails, form a superspecies, and are thought to have evolved from a single invasion from across the Atlantic.

[12] A 2010 study of molecular phylogeny further supported the possibility of specific status for R. indicus, which is estimated to have diverged from the Western forms around 534,000 years ago.

[13] The oldest known fossils of an ancestral water rail are bones from Carpathia dated to the Pliocene (1.8-5.3 million years ago).

The range of the water rail does not overlap with that of any other Rallus species, but vagrants could be distinguished from their American relatives by the lack of rufous or chestnut on the closed wing.

[13] The water rail breeds across temperate Eurasia from Iceland and Ireland discontinuously to North Africa, Saudi Arabia and western China.

[21] The Icelandic population of water rail, R. a. hibernans, became extinct around 1965, as a result of loss of habitat through the draining of wetlands, and predation by the introduced American mink.

[29] R. a. korejewi is another partial migrant, with some of the population wintering from Iraq and eastern Saudi Arabia eastwards through Pakistan and northern India to western China.

[21] In coastal areas, sea rush is common in saltmarsh breeding sites, with sedges and bur-reed dominant in somewhat less saline environments.

[31] The preferred habitat is Phragmites reedbed with the plants standing in water,[32] with a depth of 5–30 cm (2.0–11.8 in), muddy areas for feeding and a rich diversity of invertebrate species.

In addition to natural fresh or marine marshes, this rail may use gravel or clay excavations and peat workings as long as there is suitable habitat with good cover.

However, factors such as temperature, rainfall, length of shore line and extent of peat, important for some other marsh birds, were not statistically relevant.

The areas with the highest densities of the rail also had the greatest numbers of three species considered at risk in Finland, the great reed warbler, Eurasian bittern and marsh harrier.

[37] A Welsh study suggested that individual winter territories overlap, with each bird using a significant proportion of the reed bed.

[21] This species sometimes wanders well outside its normal range and vagrants have been found in the Azores, Madeira, Mauritania, the Arctic,[28] Greenland, Malaysia and Vietnam.

It swims, when necessary, with the jerky motion typical of rails,[28] and it flies short distances low over the reeds with its long legs dangling.

Large strongly-marked males are dominant in winter, when the direct aggression is replaced by sharming while standing upright on tip-toe, head jerking and bill thrusting.

The eggs are blunt and oval, smooth and slightly glossy; the colour varies from off-white to pink-buff, with reddish-brown blotches mainly at the broader end[28] that sometimes merging into a single patch.

These include leeches, worms, gastropods, small crustaceans, spiders, and a wide range of both terrestrial and aquatic insects and their larvae.

[44] Despite its skulking nature, the water rail appears to thrive in captivity when fed on animal food such as raw meat or earthworms;[45] one individual was taught to jump for worms suspended from a fishing rod.

[46] Water rails may defend a winter feeding territory, although this is smaller than when breeding, with individuals perhaps less than 10 m (11 yd) apart;[21] favoured sites may hold hundreds of birds.

[58] Parasites include the sucking lice Nirmus cuspidiculus and Pediculus ralli,[59][60] the tick Ixodes frontalis,[61] and the louse fly Ornithomyia avicularia.

[62] The water rail can be infected by the avian influenza virus[63][64] and the bacterium Borrelia burgdorferi, carried by Ixodes ticks, which is also a human pathogen causing Lyme disease.

[65] Three lice, Fulicoffula rallina, Pseudomenopon scopulacorne and Rallicola cuspidatus discovered on dead water rails in 2005 on the Faroe Islands were all species that had not been found on the archipelago previously.

[66] The parasitic flatworm Ophthalmophagus nasciola was found in one rail's nasal sinus,[67] and at least three species of feather mite have been detected on the plumage.

[69][70] The water rail's numbers are declining, but it has a large population of 100,000–1,000,000 adults and a huge breeding range estimated at 15,600,000 km2 (6,000,000 sq mi); it is therefore classed as least concern on the IUCN Red List.

In addition to the extirpation of the Icelandic race, mink have been responsible for marked declines in the populations of water rails and other ground-nesting birds in the Hebrides, where the mainly fish-eating otter was the only native carnivore.

[72] Mink and ferret eradication programmes have enabled the rail to return to islands including Lewis and Harris,[73] and further projects are ongoing or planned on the Scottish mainland.