Bitterns, which were once extinct in the UK, have been found to be at their highest levels ever recorded in England.

As a shy, secretive bird with plumage that perfectly camouflages it against its reed bed habitat, the bittern (Botaurus stellaris) can be easily overlooked.

However, the male’s booming mating call, created by pumping air through their throats and able to be heard from several kilometres away, gives its presence away and it is this distinctive call that has helped researchers count them.

The breeding population has just been discovered to be in the most rudest of health since the 1800s.

This year 140 “boomers”, or singing males, were tracked in England – an increase from just 11 in 1997.

Somerset now has the country’s largest bittern population, with 20 males located at the county's Ham Wall nature reserve alone.

The RSPB said restored quarries were also playing an important role in the bird’s revival, with 14 of the 61 sites bitterns were found in being current or former working pits.

“I’ve been working with bitterns for 10 years and it is wonderful to see how they have responded to the habitats we have restored for them," RSPB scientist Simon Wotton said.

“They’re amazing birds to watch so it is incredibly rewarding to see their numbers growing.”

Bittern numbers declined in the UK after reedbeds were drained for agriculture and birds were also hunted for food.

Red List

Nesting bitterns were lost from the countryside entirely in 1886 but began to recolonise the Norfolk Broads in 1911. The population peaked in the 1950s before falling again to leave just a handful of breeding pairs.

Conservation efforts began in 1997 to bring the species back from the brink, including the creation and restoration of wetlands across the country and the legal protection of breeding areas.

More than 300 hectares – about the size of the City of London – of new reedbed has been created since 2002 and another 350 hectares restored.

The bittern remains on the RSPB's Red List of threatened species, but conservationists say the new sites mean there will always be enough birds to boost any local populations that may struggle in the future.

Martin Harper, the RSPB’s director of conservation, said: “The bittern success story should give hope that it is possible to recover threatened species.”