A marsh harrier (Circus aeruginosus) hovering slowly above the reeds is a common sight in Vanhankaupunginlahti in the summer. The area’s waterfowl have little to fear from it, however, as its diet consists mainly of water voles, frogs and ducklings.
Marsh harriers return from their migration in April, and leave again in September. The marsh harrier does not nest in Helsinki; the individuals seen in Vanhankaupunginlahti consist of wanderers passing through the area in the spring and young birds arriving from the north in the late summer/autumn. The marsh harrier first started nesting in Finland just under a hundred years ago. Their numbers have grown in recent decades, especially in the lush inland lake areas.