Marsh harriers are slow fliers and can remain airborne at speeds of less than 32kmph (20mph). Flying at such slow speeds makes it easier for them to detect prey. Northern European and Asian birds tend to migrate, with British birds going to West Africa, but even in Britain, some birds are sedentary and do not make these migrations. Their preferred habitat is moorland, marshes, fens, reedbeds and swamps. Some species in Britain are now breeding in cereal fields. Frogs make up a large component of the marsh harrier's diet, but small mammals, reptiles, insects, eggs and nestlings are also consumed.