The organisation wants to spend the money on the islands of Egilsay, in the Orkneys, and Oronsay, in the Inner Hebrides, two of the last United Kingdom strongholds of the fast-disappearing corncrake.
Once common on farmland, the corncrakes have vanished from most of the UK since early this century because of changing habitat and breeding failure.
The species has ceased to breed in Northern Ireland, leaving the northern and western isles of Scotland as the final footholds.
Traditional farming methods continue on the two Scottish islands and the society believes that with careful management, small populations of these summer-visiting birds could be increased.
Because the birds are secretive, the only means of measuring population size is by counting the number of males giving out the distinctive rasping call of the species during the breeding season.
Seven males were heard calling on Egilsay last year and the RSPB plans to buy Onziebust Farm, covering a quarter of the island, in the hope it can more than double the number.
Oronsay had three calling males last year and the society, which aims to lease the island, believes that by improving the habitat the number could be increased.
There were just 570 calling male corncrakes found in the UK last year and in the 25 years to 1993, the national population declined by more than 50 per cent.