About five; see text The common moorhen (Gallinula chloropus), also known as the waterhen, is a bird species in the rail family (Rallidae).

The common moorhen was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae.

[2][3] The common moorhen is now one of five extant species placed in the genus Gallinula that was introduced in 1760 by the French zoologist Mathurin Jacques Brisson.

[7][8][9] The final species in the genus, the dusky moorhen G. tenebrosa of Australasia, has also been considered conspecific by some authors in the past.

[11] A "watercock" is not a male "waterhen" but the rail species Gallicrex cinerea, not closely related to the common moorhen.

The frontal shield of the adult has a rounded top and fairly parallel sides; the tailward margin of the red unfeathered area is a smooth waving line.

[10] In the related common gallinule (Gallinula galeata) of the Americas, the frontal shield has a fairly straight top and is less wide towards the bill, giving a marked indentation to the back margin of the red area.

The common moorhen gives a wide range of gargling calls and will emit loud hisses when threatened.

[13][14] This is a common breeding and resident bird in marsh environments, rivers, well-vegetated lakes and even in city parks.

The Royal Society for the Protection of Birds in the United Kingdom has the common moorhen classified as one of its 103 species whose conservation status is of moderate concern[19] due to its recent population decine.

The common moorhen is one of the birds (the other is the Eurasian coot, Fulica atra) from which the cyclocoelid flatworm parasite Cyclocoelum mutabile was first described.

[29] Abdul hamid II, the then-Sultan of the Ottoman Empire, was known to have a special interest in the Moorhen, which was found in almost all bodies of freshwater in the land which now constitutes Turkey.