The Eurasian Marsh Harrier is a common and widespread winter visitor. The first birds arrive from Central Asian breeding grounds in August and most depart in April, although stragglers, presumably immature birds, can be seen as late as June. The great majority are immature or female birds with chocolate brown bodies and usually varying amounts of yellowish buff on the head and shoulders. Full adult males, which take up to four years to mature, are greyer and much rarer.

Marsh Harriers need reedbeds in which to roost and frequently feed over them. But they also feed over agricultural crops so may be seen some distance from water. The roosting birds begin gathering well before dusk and circle the chosen roost site for up to an hour before they settle. This is the best time to count them.

The Marsh Harriers low quartering and sudden pouncing is characteristic and several birds will hunt together. They feed mainly on small birds and amphibians in the reeds and on open ground but will also hunt small duck and Coot on open water. A low flying Marsh Harrier is guaranteed to flush the flocks which are what the hunter wants, as it can then target those that are not fully fit.