Ornithologists in the United Kingdom are concerned at the disappearance of most of the bitterns which hatched this year.

The bittern is one of the UK's rarest and most threatened breeding birds, and has been in decline for more than 40 years.

In 1954, there were about 80 male birds, in seven counties. But by 1997, there were only 11, and just 13 the following year.

This year saw a modestly encouraging increase to 19 breeding male birds. At the end of the breeding season, 20 young bitterns, all hatched this year, were caught and fitted with radio transmitters.

Casualty

Now, the Royal Society for the Protection of Birds (RSPB) says, 16 of the 20 have vanished, and it is asking birdwatchers everywhere to be on the lookout for them.

Even radio tags cannot find them

Of the tagged birds that have been found, one had been injured. It appeared to have flown into overhead wires, and did not survive.

A second tagged bittern was spotted during the British Birdwatching Fair in August, and was reported to be in good condition.

Dr Ken Smith, the RSPB's head of aquatic research, said: "We desperately need to know where our young bitterns go to".

"Each bird could turn up in almost any area with water and reeds or other thick aquatic vegetation."

Vulnerable to pollution

Bitterns are secretive members of the heron family which depend on reedbeds, where they breed and feed mainly on fish and amphibians.

They are vulnerable to pollution, and also suffer badly in severe winters, with many dying when their wetland habitats freeze over. Some move then to gravel pits, rivers and lakes.

The RSPB is working to create new reedbeds in the parts of East Anglia where most British bitterns live. There is also a tiny population in Lancashire.