Numbers of corncrakes dropped in 2008
The public are being asked to listen out for the distinctive call of the corncrake to gauge the spread of the rare migratory bird.
RSPB Scotland said the species was difficult to spot and so more likely to be heard. Its numbers fell last year.
The charity described its call as sounding like a credit card being drawn across a plastic comb.
Good concentrations were thought to exist on Lewis, North and South Uist, Tiree and Coll.
However, the RSPB hope to find the bird has spread beyond these strongholds.
Mark O'Brien, advisory manager at 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 years slight slump in numbers was just an anomaly in what has been an otherwise fabulous success story for this species.
"The recovery of this plucky wee bird has, in a large part, been down to previous agri-environment schemes."
The species is a long-distance migratory species that winter in sub-Saharan Africa and come to the UK in summer to breed.
Their favoured habitat is tall grasses and herbs, particularly hay and silage meadows.
However, in the late 19th Century the mechanisation of farming led to crops being harvested quicker and corncrake population plummeted and became restricted to the Hebridean islands on the west coast of Scotland.
In 1993, the British population was estimated at just 480 males heard calling for mates.
But by the last national survey in 2003 this figure had almost doubled to 832 males.
Since then annual counts have shown that the population increase continued throughout the 2000s, and in 2007 the population in its Scottish strongholds hit a high of more than 1,270 calling males.
However, this number declined in 2008 by 8% to 1,140.
The survey seeks to find out whether the recovery of corncrake has resumed this year and to establish whether the decline of 2008 was a temporary blip, or part of a more sustained pattern.