Ailsa Craig, 10 miles off Girvan in the Firth of Clyde, is back on the market for £1,500,000. The 245-acre volcanic plug, the source of the granite used for curling stones, is home to colonies of protected birds.
However, those rare selling points have not been enough to persuade anyone to meet the original asking price of £2.5m.
It was put on sale in 2011 by Charles Kennedy, the eighth Marquis of Ailsa, but failed to attract a buyer. Any new owner would be able to command a rent of £26,000 a year.
Known as Paddy's Milestone, because it lies on the route from Belfast to Glasgow, Ailsa Craig is home to 70,000 birds, including gannets, guillemots, razorbills, kittiwakes and herring gulls.
The RSPB, which has managed the island for more than 25 years, said previously it was not in position to buy the island.
Dr Dave Beaumont, RSPB Scotland regional reserves manager, said: "We rely on the generosity of individuals and grants from public and commercial bodies for our income, and as a charity we need to make sure these funds go as far as they can.
"Our lease on the island runs until 2050, and this currently allows us to achieve our conservation goals. We will be watching the sale closely and will be very keen to work with the new owners to further the conservation of this magnificent island."
Ailsa Craig is about two miles wide and rises 1100ft above the sea. The firm that makes curling stones has a lease to extract "blue hone" granite.
Potential new owners would also inherit three cottages, a ruined castle and a lighthouse.
It is being sold by Vladi Private Islands, which has sold 2000 islands worldwide since it was set up more than 30 years ago.