Marsh harriers have always passed through Fowlmere on spring and autumn migration, but we have seen them more frequently as the national breeding population has increased. Assessing the age of juveniles (born this year) in autumn is relatively easy - they are very dark brown, usually with a golden crown, and a full set of wing feathers - but older birds can be more difficult.
Last week a marsh harrier arrived that most people thought was an adult female. The brown parts of the plumage were very faded and tatty, and the crown and nape were almost white. Marsh harriers start their moult in late summer, so an adult can be expected to have worn plumage at this time of year, and have gaps in the trailing edges of the wings where old feathers have been dropped and new ones are growing through. Our bird was showing such gaps, so it could have been an adult female. However, in flight the truth was revealed. As can be seen from the photos below (taken rather hastily by Geoff Harries, and far below his usual standard), the inner three (new) primaries are actually grey with dark spots near their tips. This bird is in fact a male, undergoing its first complete moult from its worn juvenile plumage to its second year plumage.
If this bird stays around, we can expect to see more of the grey feathers as they replace the next primaries, and then the outer primaries will be replaced by black new ones. In fact, the harrier is likely to migrate before the moult is complete, in which case it will probably suspend the moult and retain some of the old outer primaries until it reaches its winter quarters in Africa.