Scientists say the discovery justifies the decision to eradicate the population of rats on the rocky outcrop.

In the 1870s, when puffins used to fly over the island, their huge numbers were said to cause a "bewildering darkness".

But humans brought rats by mistake to Ailsa Craig in the 19th century, apparently carried on ships supplying the newly-built lighthouse.

The loss of bio-diversity that occurs when predators like rats are introduced to offshore islands is tragic

Professor Pat Monaghan Glasgow University

Puffins were particularly vulnerable because of their burrow nests and their numbers declined from tens of thousands to only a few hundred by the 1930s.

None had bred for more than half a century.

In 1991, a project began in collaboration with the University of Glasgow, the island's owner, the Marquess of Ailsa and Scottish Natural Heritage with advice from the pest control company Rentokil.

Several tonnes of poison were flown there by navy helicopter and more baiting took place in 1992. No rats have been recorded on the island since then.

'Wonderful sight'

Dr Bernard Zonfrillo, from the university, said: "It's wonderful to see the puffins back breeding.

"At least two pairs were seen carrying fish to their chick on Ailsa Craig this summer. Given time and left to their own devices, I hope their numbers will gradually increase."

Professor Pat Monaghan, from the university's department of environmental and evolutionary biology, added: "The loss of bio-diversity that occurs when predators like rats are introduced to offshore islands is tragic."

Scientists say other seabirds and even rare plants, reptiles, and some other mammal species are flourishing on the island.