The island decided to get rid of its cats in 1980 but instead of reversing the declining numbers of petrels, their population dwindled further, the researchers wrote in the latest issue of the Proceedings of the National Academy of Sciences.

While 32 percent of breeding burrows fledged a chick when both cats and rats were around, only 9 percent produced chicks after cats were removed from their breeding areas.

"The cats were suppressing the rat numbers, so the removal of the cats allowed rat numbers to increase and as rat numbers increased ... they (hunted) petrels more heavily," Matt Rayner at University of Auckland's School of Biological Sciences said in a telephone interview.

The island finally got rid of its rats in 2004 and the breeding rates of petrels shot up to 59 percent.

But at lower altitudes, petrel breeding rates were little changed even after the rats were removed -- which the researchers attributed to the availability of other food sources for the rodents lower on the slopes.

Cook's petrels used to be found everywhere in New Zealand, but they disappeared from the mainland in the 19th century due to encroaching human developments.

The size of pigeons, petrels are now found in only one other island in New Zealand, on Whenua Hou at the southern tip.

"They are poorly adapted to predation by mammals but can live up to 30 years. They live a long time but breed very slowly ... one egg every couple of years," Rayner said.

It was important for conservationists to fully understand ecological systems before carrying out eradication exercises, Rayner said.