How have marsh harriers, scarce in the 1970s, bounced back over the last 50 years?

Marsh harrier - Les Bunyan (rspb-images.com)

One of our supporters, Judith McDonald, kindly posted us some old issues of Bird Life, the quarterly magazine for the RSPB’s junior branch from 1965 to 2000.

I was fascinated to find an article on marsh harriers by Minsmere’s pioneering warden Bert Axell in the October – December 1971 edition.

Copies of Bird Life magazine - Jamie Wyver

Bert wrote: “Of the three harrier species which breed in Britain, the marsh harrier is by far the rarest. If you have visited the RSPB reserve at Minsmere, Suffolk, in recent years and seen the two or three pairs which breed there, you will, in fact, have been looking at rather more than half of this country's population of these magnificent raptors.”

Fifty years later, you have a much greater chance of seeing a marsh harrier. Over 600 pairs now nest in the UK and many more now spend the winter here rather than flying south. 

The other harrier species Bert refers to are the hen harrier and Montagu’s harrier. Hen harriers are still a long way from establishing a stable population. The government’s own study, published in 2019, found persecution to be the main factor limiting their recovery in the UK. And in 2021 no Montagu’s harriers bred here. So it’s safe to say that if you’re in the UK, watching a harrier over a large reedbed, it’s the marsh harrier you’re most likely to be looking at.

Return of the marsh harrier

So how has this bird made such an incredible comeback?

Marsh harrier at RSPB Titchwell Marsh - Les Bunyan (rspb-images.com)

In 1971, Bert reported that the threats facing marsh harriers were loss of habitat, human disturbance, and pesticides. Draining of wetlands was a historical problem and already many of our reedbeds and marshes had gone. Persecution would have been an issue too: all raptors have been heavily persecuted.

In East Anglia around that time there would also have been large numbers of coypu (like the one picture below) munching their way through reeds. These huge rodents had escaped from fur farms and built up significant populations in the area, destroying what was left of the harriers’ reedbed home. By the end of the 1980s, these had been eradicated. But Bert mentions that even when the numbers of coypu were going down, the numbers of harriers still weren’t really going up.

Coypu - Dale Sutton (rspb-images.com)

Pesticides were a bigger problem, building up in the birds’ bodies from their small prey. This caused the eggshells of birds of preys to thin, reducing the chances of the chicks surviving. The banning of the insecticide DDT in the 1980s helped with the recovery of several species of birds of prey, including marsh harrier.

Perhaps the biggest change for the UK’s marsh harriers since Bert was warden at Minsmere has been the creation and restoration of many reedbeds across the UK. For example down the road from Minsmere, at Lakenheath Fen, we’ve created 400 hectares of wetland habitat, including reedbeds, on former carrot fields. We’re making one of the UK’s biggest freshwater reedbeds at Ouse Fen in Cambridgeshire in partnership with Hanson UK; by 2030 there will be 460 hectares of reedbed, part of a 700-hectare wetland wildlife haven.

Marsh harriers and bitterns

Bert also describes the complex relationship marsh harriers have with bitterns, their reedbed neighbours. I’ve often seen a bittern fluffed up like a bottle brush (as in the photo below) to appear larger when a harrier is passing overhead. But what I hadn’t considered is that like most herons, bitterns will happily gobble up anything they find moving on the ground, including marsh harrier chicks.

Fluffed-up bittern at RSPB Minsmere - Kevin Sawford (rspb-images.com)

“Once a family of four bitterns, an adult and three smaller ones, all leapt out of the reeds together to attack a male harrier diving on them to defend its own nest – a spectacular sight that boded ill,” Bert writes.“When, a few days later it was plain that the harriers had deserted their nest, I inspected it to see what had happened. The nest contained one broken infertile egg and the mangled remains of a young harrier about three weeks old. Only six yards away I found a bittern’s nest which had evidently been recently occupied by young. How had a harrier come to invite disaster by nesting so close to the already-established nest of one of its mortal enemies?”

Perhaps now, with more reedbed space to choose from, both species can thrive without such disastrous disputes.

Marsh harriers today

Today, you’re most likely to see marsh harriers in eastern and south-east England, with some in the north-west, south-west and Scotland. You’ll find them in places like the nature reserves in East Anglia mentioned above, and at other RSPB nature reserves elsewhere in the country that contain reedbeds such as Cors Ddyga and Leighton Moss.

Marsh harrier at RSPB Minsmere - Ben Andrew (rspb-images.com)

Back at Minsmere between eight and 12 pairs of marsh harriers now nest each summer: hopefully Bert would be proud.

Reedbed at RSPB Minsmere - Kelly Thomas (rspb-images.com)

Minsmere celebrates its 75th anniversary next year and alongside avocets, bearded tits, bitterns and stone-curlews, marsh harriers remain one of Minsmere’s ‘Big Five’. However, this special site is currently threatened by plans for Sizewell C, a development which could have a negative impact on marsh harriers and many other species. Find out more on our Love Minsmere page.

More about the historical decline of birds of prey

Images 

Marsh harrier at RSPB Titchwell Marsh - Les Bunyan (rspb-images.com)
Copies of Bird Life magazine - Jamie Wyver
Marsh harrier at RSPB Titchwell Marsh - Les Bunyan (rspb-images.com)
Coypu - Dale Sutton (rspb-images.com)
Fluffed-up bittern at RSPB Minsmere - Kevin Sawford (rspb-images.com)
Marsh harrier at RSPB Minsmere - Ben Andrew (rspb-images.com)
Reedbed at RSPB Minsmere - Kelly Thomas (rspb-images.com)

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