There are only 46 males in the country

The Royal Society for the Protection of Birds (RSPB) says the Norfolk population is down from 19 to 11 males.

The year's count for the country shows a drop in numbers to 46 compared with 55 last year, with Suffolk recording the highest number at 20.

A Bittern rescue plan began in the mid nineties after a dramatic decline in their numbers to just 11 in 1997.

Lowland marshes

The bittern's fortunes in some counties continued to improve with Dorset recording its first booming bittern in recent years, while Cambridgeshire doubled its population from two to four.

There was disappointment for researchers in Wales, Kent and Somerset, where last year's survey had recorded one, three and one booming bittern respectively but this year none were found.

Dr Ken Smith of the RSPB said: "In spite of this small setback the recovery of the breeding populations is a conservation success."

The bittern, best known for its booming call, was once numerous in the fens, but is now only found in a handful of areas.

It is dependent upon large tracts of wet reedbed to find sufficient food, principally fish and amphibians.