The Western Marsh-harrier (Circus aeruginosus) is a diurnal predatory species, which can be found mostly in the eastern part of the Danube's Delta Biosphere Reserve. A total of 25% of the European population of Western Marsh-Harrier nests in the Danube's Delta. In Romania, the Western Marsh-Harrier is a strictly protected species.

It is a migratory species, which will migrate during late fall to winter in the north of Africa. Males are the first to leave, followed by the females. The female has a chocolate coloured plumage, with yellow spots on its head and shoulders, whereas the male has a grey backside and a red abdomen.

The Western Marsh-Harriers have long tails, which help them during flight. They hunt in wide open areas.

Unlike other predatory species, the Western Marsh-Harriers nest at ground level. They prefer areas with compact reed populations.
The Western Marsh-Harrier does not feed on a specific nutrient. It feeds on a wide variety of species from the trophic chain, from amphibians to reptiles.

This species is monogamous, but there have been noted a few cases of polygamous males. Even so, the chicks do not suffer from food deprivation. The male brings materials and the female builds the nest. The pair also has fake nests used to rest. During one season, a pair lays 4 or 5 eggs, with a chance of survival of 40%. One week after the chicks hatch, they will hide in the reed.