The bittern 'boom' is heard up to 5km away

The species appears to be recovering well from its near extinction in 1997, according to joint research by the Royal Society for the Protection of Birds (RSPB) and English Nature.

Numbers of this extremely rare brown heron have reached at least 42 booming males - four times the level recorded six years ago.

The RSPB says the figures are a triumph for conservation measures aimed at arresting the decline.

Bittern numbers had dropped because their reedbed habitat was being drained, destroyed or neglected.

Life as a bittern The bittern is a stocky, thick-necked heron, generally golden brown with distinct black markings and dark crown The male boom marks territory and attracts females Each male has up to five females and rivals are stabbed to death When food is short they become cannibals, eating the smallest bird in a brood The boom is the lowest-pitched and the most far-carrying mating call by any European bird

Numbers declined sharply from the 1950s, when there was a peak of around 50 booming males, to just 11 booming males in 1997

That prompted efforts to create new beds and preserve the existing ones.

A �4m rescue plan was launched earlier this year, to continue the work in protecting the bittern.

Eight conservation groups and three county wildlife trusts are working on the plan, with more than half the cash coming from the European Union's Life-Nature programme.

Gillian Gilbert, RSPB bittern ecologist, said: "The bittern is still so rare that we know each of these birds individually.

"But these results are extremely encouraging and point to the achievements of recent conservation work."

Male bitterns are well known for their booming call, which sounds like a distant foghorn and can be heard 5km away.

Mr Gilbert said the increase in bittern numbers was reflected by visitor numbers at some wildlife reserves.

At Minsmere Nature Reserve in Suffolk, home to eight booming bitterns, business was up to 60,000 visitors a year, he said.