The western marsh harrier (Circus aeruginosus) is a large harrier, a bird of prey from temperate and subtropical western Eurasia and adjacent Africa. It is also known as the Eurasian marsh harrier.

Formerly, a number of relatives were included in C. aeruginosus, which was then known as "marsh harrier". The related taxa are now generally considered to be separate species: the eastern marsh harrier (C. spilonotus) and the possibly distinct Papuan harrier (C. (s.) spilothorax) of eastern Asia and the Wallacea, the swamp harrier (C. approximans) of Australasia and the Madagascar marsh harrier (C. maillardi) of the western Indian Ocean islands.

The western marsh harrier is often divided into two subspecies, the widely migratory C. a. aeruginosus which is found across most of its range, and C. a. harterti which is resident all-year in north-west Africa.

Description

The western marsh harrier is 43 to 54 cm (17 to 21 in) in length, has a wingspan of 115 to 130 cm (45 to 51 in) and a weight of 400 to 650 g (14 to 23 oz) in males and 500 to 800 g (18 to 28 oz) in females. It is a large, bulky harrier with fairly broad wings, and has a strong and peculiar sexual dichromatism. The male's plumage is mostly a cryptic reddish-brown with lighter yellowish streaks, which are particularly prominent on the breast. The head and shoulders are mostly pale greyish-yellowish. The rectrices and the secondary and tertiary remiges are pure grey, the latter contrasting with the brown forewing and the black primary remiges at the wingtips. The upperside and underside of the wing look similar, though the brown is lighter on the underwing. Whether from the side or below, flying males appear characteristically three-colored brown-grey-black. The legs, feet, irides and the cere of the black bill are yellow.

The marsh harriers are birds of prey of the harrier subfamily. They are medium-sized raptors and the largest and broadest-winged harriers. Most of them are associated with marshland and dense reedbeds. They are found almost worldwide, excluding only the Americas.

Until recently two species were generally recognized: the marsh harrier (Circus aeruginosus) and the African marsh harrier (C. ranivorus). The marsh harrier is now usually split into several species, sometimes as many as six. These are the western marsh harrier (C. aeruginosus), eastern marsh harrier (C. spilonotus), Papuan harrier (C. spilonotus spilothorax or C. spilothorax), swamp harrier (C. approximans), Réunion harrier (C. maillardi maillardi or C. maillardi) and Madagascar marsh harrier (C. maillardi macrosceles or C. macrosceles).

At the beginning of the 20th century, the Marsh harrier was hunted to extinction in the United Kingdom. After being reintroduced from other regions, its population steadily increased until DDT threatened it, along with other raptors, in the 50's and 60's. Since, the population has slowly and steadily increased.