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.
Two subspecies are recognised: C. s. spilonotus of east Asia, and C. s. spilothorax, of New Guinea and the islands nearby, which is sometimes thought to be a separate species and is often called the Papuan Harrier. The Swamp Harrier (C. approximans) at about 50 to 58 cm is slightly larger than C. spilonotus, often a little darker, and has less heavily barred wings and tail. It is commonly found in suitable habitat anywhere in Australasia, particularly in the higher rainfall areas to the east, south-east, and south-west, of Australia and throughout New Zealand, but also in the tropical north of Australia and the island groups of New Caledonia, Vanuatu, Fiji, Tonga and the Society Islands. It is also known as the Australasian Harrier or Pacific Marsh Harrier. The Madagascar Marsh Harrier or Madagascar Harrier (C. maillardi) is found on the Indian Ocean islands of Madagascar, Réunion and the Comoros. It is sometimes split into two species: Madagascar Harrier (Circus macrosceles) and Réunion Harrier (C. maillardi). The African Marsh Harrier (C. ranivorus) is a distinct non-migratory species that has not usually been included as a subspecies of C. aeruginosus. It inhabits southern and eastern Africa.