They may be one of Britain's rarest bird – but they're also one of the loudest.

Titchwell RSPB Reserve has been echoing to the booming calls of the engimatic bittern over the last few days.

Wardens and visitors believe there are two males performing the birds' loud mating call – along with a repertoire of grunts.

The reserve, near Hunstanton, is one of the ultra-rare bird's last strongholds.

Warden Paul Eele said: 'This has been the only noise we have had in the last two seasons. There has been a change this year, there seems to be a male advertising his presence. They can do that when trying to defend themselves.

'But I have heard one bittern grunting and another was booming behind me the other day, so my hunch is there is a second male.

'It's quite exciting.'

The bittern is a member of the heron family, which is secretive and very difficult to see because it is well camouflaged and lives in dense reedbeds.

Mr Eele said a survey would now be carried out to assess the reserve's population – meaning an early start for some.

He said: 'The survey will mean that we will get to the site early as bitterns are at their most active at early dawn.

'We will listen and record what we hear and see what other birds are on the site.

'I think people like the mystery of the bittern and the difficulty in finding them. They make such a surreal sound, it's great.'

The bittern is among Britain's most threatened bird species, with less than 50 breeding pairs living in East Anglia and Lancashire.

After becoming extinct in 1886, the bird reappeared in the Norfolk Broads in 1911. Numbers initially increased until the 1950s, but then declined again until 1997, when intensive conservation efforts were launched.

Twitchers rate Titchwell as one of the best spots in the country when it comes to the chances of seeing one.

Have you heard bitterns booming this spring? E-mail natalie.copeland@archant.co.uk