It was just a feather: but in the tropical paradise of the Seychelles, the discovery of parakeet plumage has put environmentalists in a flutter, with a foreign invading bird threatening the national parrot.

In the steamy and thick jungles of Praslin, one of 115 islands in the Indian Ocean archipelago of white sand beaches ringed by palms, the authorities are worried after the warning sign of a green feather from a ring-necked parakeet was discovered.

If not stopped, it threatens to wipe out the country's endangered black parrot, listed as "vulnerable to extinction" on the Red List of the International Union for Conservation of Nature (IUCN).

"Based on the characteristics that I could see, the size and specifics... It is indeed a feather from the ring-necked parakeet," said Vilna Accouche, a scientist with the Seychelles Islands Foundation (SIF), which helps conserve the palm-filled Vallee de Mai national park, a UNESCO-listed World Heritage Site and main home to the parrots.

The vivid green parakeet with a bright red beak, originally a domestic bird from India but which escaped its cages in the 1970s and now breeds in the wild on the main island of Mahe, may appear harmless.

But it is a direct rival to the endemic black parrot, the "most noteworthy species" in the park and which is "totally dependent" on the palm forest there, according to UNESCO.

The black parrots live on Praslin, some 45 kilometres northeast of Mahe, and previously hoped too far to fly for the parakeet.

That isolation has left the parrots unused to competition, and vulnerable to infections from outside.

Rats, a major pest, as well as cats have until now been its main threat, with some nest boxes protected by rat traps.

But now the green parakeet appears to have flown the island divide and is threatening the parrot too.

As well as munching the same food as the parrots, the parakeets could bring with them diseases.

"The biggest danger posed by the arrival of the ring-necked parakeet on Praslin is that it could carry beak and feather disease, and spread them to the black parrot," said Accouche.

For the tropical islands, where tourism is the mainstay of the economy, preserving its unique nature is vital.