Experts hope the bittern population will reach 100 males by 2020

In 1997 only 11 bitterns were found during a UK-wide survey, but this year experts counted 55 bitterns at 30 sites, including Norfolk and Suffolk.

An RSPB spokeswoman said decades of drainage and pollution had destroyed most of the their habitat by 1997.

But work to restore reedbeds has helped to protect the bittern, she added.

Species 'still threatened'

The joint RSPB and English Nature survey counted 19 bitterns in Suffolk, 19 in Norfolk and two in Cambridge.

Bitterns were also found at sites in Humber, Lancashire, Wales, Kent and Somerset.

The RSPB's bittern researcher, Dr Gillian Gilbert, said: "Bitterns are now recovering at a faster rate than anyone dared hope.

"Only a few years ago its numbers were in steep decline."

Since 1997, Broads Authority has been working with conservation groups to protect reedbeds as part of the European Union funded Life Nature project.

The work has also helped preserve wetland species, from the otter to the marsh harrier, the RSPB added.

But the RSPB's head of aquatic research, Dr Ken Smith, said: "We shouldn't be complacent as pressures still threaten this vulnerable bird. For example, half the sites holding booming bitterns this year are at risk from rising sea levels."