RATS and foxes are swimming 200m to gorge on rare birds in a sanctuary off Outer Harbor.
Ecologists have responded to the invasion of Bird Island with months of intensive baiting, which they hope will result in a record bird breeding season this summer.
Seascapes co-ordinator Damian Moroney, from the Environment Department, said the 1.5km-long island was a sand spit which had formed above the water level in the 1970s and was now “an incredibly important breeding site”.
December’s monthly census recorded almost 600 bird nests, including those of the endangered fairy tern.
There were an estimated 1500 fairy terns in southeastern Australia and 160 of them, more than 10 per cent, were nesting on Bird Island.
“We’ve now focused our attention on these endangered birds and what we need to do,” Mr Moroney said.
He said the volume of bird activity on the island lured rats and foxes to make the crossing from the mainland.
For several years, baiting had proved largely ineffective until surveillance cameras revealed the rats were too big to reach inside the commercial bait stations.
“We’ve got very big rats out there,” he said.
After much research, 128 oversized bait stations were developed and placed on the island in October.
Volunteers reloaded the stations up to three times a week until rat activity reduced and three fox dens also were fumigated.
Other species on the island in December included silver gulls, white ibis, crested terns, caspian terns, black-faced cormorants and pelicans, with 142 nests.
“It’s now the second-most important permanent pelican breeding site in SA after the Coorong,” Mr Moroney said.
Mr Moroney said the island was “an amazing site” but the public was discouraged from going ashore.
Startled birds could abandon their chicks and visitors risked trampling the eggs of the beach-nesting fairy terns, which blended into the sand.
“Anyone with any soul would have to appreciate just what’s happening out there but the whole thing can be destroyed by human interference,” he said.