Bitterns have been found nesting in the Fens for the first time since before the Second World War bringing fresh hope of a 'boom' in numbers of one of Britain's most threatened birds.

Bitterns have been found nesting in the Fens for the first time since before the Second World War bringing fresh hope of a 'boom' in numbers of one of Britain's most threatened birds.

Conservationists are particularly excited by evidence of four nests involving three different breeding females at Kingfishers Bridge, a privately-owned, 150-acre wetland site near Wicken Fen in Cambridgeshire, making landowner Andrew Green the first since 1938 to have nesting bitterns in the area.

The site, which in 1995 was Grade 1 arable land, has been partially converted to reedbed by Mr Green, with help from wildlife consultant Roger Beecroft. It is the first time that bitterns have been recorded nesting in a newly-created reedbed; the bird's preferred habitat.

RSPB conservation director Dr Mark Avery said: “The spread of the bitterns into areas of newly-created reedbeds is a testament to all those involved with habitat creation, especially in the East Anglian Fens.

“Reedbed sites occupied by more than 40 per cent of male bitterns this year are at risk. Rising sea levels threaten to inundate eight freshwater reedbeds, increasing salinity and making them unsuitable for this sensitive bird.

“Encouraging bitterns to nest in newly-created sites away from the coast helps to buffer these sensitive birds against the impacts of climate change and will help safeguard their future in Britain.”

Mr Green said he was delighted with the discovery. He said: “We think the key to the project's success is dependent upon a number of factors; good evidence-based habitat management; pure water and control of water levels; the creation of a rich fishery, providing excellent feeding opportunities for bitterns; and the rigorous control of foxes and mink.

“We are grateful to the RSPB for its research into bitterns, which has helped design an ideal site, and for the supply of reed seeds.”

A survey of the birds has shown that their recovery took a tentative step forward this year with male bitterns recorded at more sites than any other year since 1990, when detailed annual monitoring began.

Extinct in Britain between 1886 and 1911, the bittern reached a second low point in 1997, when the bird's population dropped to 11 males.

However, 10 years on, surveyors from the RSPB and Natural England recorded a minimum of 51 male bitterns across 33 sites, giving encouragement to everyone involved in the birds' conservation.

RSPB research in the 1990s showed that reedbeds containing bitterns were drying out, leaving the bird with little suitable habitat leading to a perilous population decline.

Historically, researchers have relied on listening for the male's 'booming' call to help gauge the population. But more recently scientists have also identified locations of nests with chicks in the hope of providing a more accurate reflection of the nesting success of bitterns.

As a nesting bird in the UK, the bittern is confined to England with East Anglia containing more than three-quarters of numbers of 'booming' males and a minimum of 24 nests with young.

The Bittern Monitoring Programme is jointly funded through Action for Birds in England, a conservation partnership between Natural England and the RSPB.