The corncrake, a migratory bird which over-winters in Africa, was once common across the UK, but was hit by changes in agriculture in the 19th and 20th century, the RSPB said.

Mechanised mowing allowed hay-making to be completed more rapidly, destroying the tall grass and meadow habitats the bird used for nesting and leading to a collapse in the population. Corncrakes became restricted to Hebridean islands on the west coast of Scotland where crofters were much slower to turn to mechanised agriculture.

Recent efforts by conservationists, farmers and crofters helped the corncrake population double from just 480 calling males in 1993, when the RSPB launched a recovery programme for the bird, to 832 calling males in 2003. And a reintroduction scheme since 2002 in England, near Peterborough, has sought to bring the corncrake back south of the border, and last year recorded 14 calling males.

The population in the Scottish strongholds hit 1,270 in 2007, but dipped to 1,140 in 2008.

A national corncrake survey organised by the RSPB, the Northern Ireland Environment Agency and Scottish Natural Heritage, is the first full survey since 2003.

It aims to find out whether the recovery of corncrake numbers is continuing, and whether 2008 was a temporary blip in an upward trend.

Mark O'Brien, of RSPB Scotland, said: "This is the first full survey since 2003 and numbers have undoubtedly increased since then, but we are really interested in seeing if last year's slight slump in numbers was just an anomaly in what has been an otherwise fabulous success story for this species."

But corncrakes are difficult to spot, because they are well camouflaged, shy and hide in long vegetation, so the best way to identify them is by the distinctive "crek crek" calls made by mating males.

The RSPB said the sound was like a "credit card being drawn across a plastic comb", and the birds were particularly vocal throughout June, especially at night.

The conservation charity is urging anyone who hears the distinctive call to report it, so it can be verified and recorded as part of the survey.

Anyone who hears a corncrake calling is asked to call the RSPB on:
England and Wales - 01767 680551
Scotland - 0131 311 6500
Northern Ireland - 028 9049 1547