BTO: Marsh harriers return to reedbeds

Marsh harrier (Circus aeruginosus) by Amy Lewis ANL-141011-141915001
Marsh harrier (Circus aeruginosus) by Amy Lewis ANL-141011-141915001

Although the marsh harrier is ‘amber-listed’ on the Birds of Conservation Concern list – www.bto.org/volunteer-surveys/birdtrack/bird-recording/birds-conservation-concern – its fortunes seem to be on the up, the breeding range having doubled over the last 20 years.

Suffolk’s coastal marshes and reedbeds lie within the core of this range and the marsh harrier is now a familiar sight, seen drifting low across the reeds on long narrow wings.

The species has something of a chequered history, having declined to extinction at the end of the 19th century before increasing again. There was then another collapse in the breeding population during the late 1960s, leaving a single breeding pair – in Suffolk – to retain a toe-hold here in England. The increase that has followed reflects reduced persecution of nesting pairs, more favourable management of favoured habitats and an end to the use of organochlorine pesticides, which reduced the breeding success of a number of our birds of prey.

Two-thirds of the breeding population are to be found in Suffolk, Norfolk and Lincolnshire, underlining how the marsh harrier has spread out from those first few breeding pairs.

One of the more interesting aspects of the species’ success has been the use of arable crops, the birds now utilising the cover provided by oil-seed rape and winter cereals. Large reedbeds, however, provide a more productive habitat and can support several breeding females. Male marsh harriers often pair with more than one female, a behaviour termed ‘polygamy’ by ornithologists. Polygamous pairings will only succeed where sufficient prey is available, allowing the male to support more than one female and her brood of 2-8 chicks. It is the male who provides all of the food while the female is incubating and for the first week or so after the chicks emerge from their eggs. Prey captured by the male may be passed to the female in flight, which can make for dramatic viewing.

While marsh harriers may be seen throughout the year in Suffolk, it is during the winter months that they are seen most readily, as individuals gather together at favoured roosting sites. The roost at Lakenheath Fen, for example, which sits on the border between Suffolk and Norfolk, can attract more than two-dozen marsh harriers, not to mention other bird of prey species. The birds arrive during early afternoon, their numbers increasing towards dusk, when they drop down into the reedbeds to roost.

That these birds now seem to be doing so well reflects the hard work of dedicated volunteers, whose efforts during the 1970s secured those first few breeding attempts from egg collectors and others wanting to do the birds harm. Their legacy is that we can now enjoy this wonderful bird of prey, the haunter of our reedbeds and coastal grazing marshes.