Bittern: medieval banqueting dish that died due to hunting and loss of habitat - Telegraph.co.uk

But experts say males have an "amazing beatbox ability", in which they fill their gullets with air and released to make a booming "song".

Sometimes its sounds can be heard several miles away, enabling scientists to determine the bird's population.

But it still is threatened by the reduction in availability and quality of "Phragmites-dominated swamps and other marshes due to drainage and abandonment of traditional uses for reed beds", the RSPB said.

It also faces threats including sea level rise, where freshwater sites along the coast could be inundated by saltwater.

"In the past, continued drainage of land for agricultural uses and excessive water abstraction in the catchment of reed beds reduced the area of reed beds in the UK," a spokesman said.

"The main current threat to the habitat comes from neglect and lack of management, which allows reed beds to dry out and become unsuitable for bitterns."

The bittern is listed on Schedule 1 of the Wildlife & Countryside Act 1981, which "affords it special protection at all times".

"It is an offence to take, injure or kill a bittern or to take, damage or destroy its nest, eggs or young," the RSPB said.

"It is also an offence to intentionally or wrecklessly disturb the birds at or close to their nest during the breeding season."

People convicted face a maximum sentence of up to £5,000 per offence and/or a prison sentence of up to 6 months.