The sighting of two rarely seen tropical birds in Hong Kong could be down to climate change, bird experts said Saturday. The birds - a great frigate and the white-tailed tropicbird - were both spotted around Po Toi, Hong Kong's southern most island, over the last month.

It was the first time the white-tailed tropic had ever been spotted in Hong Kong and only the fourth sighting of the frigate.

Both birds are usually seen in more tropical climates such as the Pacific and Indian Oceans.

Hong Kong Bird Watching Society Chairman Cheung Ho-fai told the South China Morning Post that their appearance could be attributed to climate change and that the nearest nesting colony of frigate birds was in Hawaii.

"The birds are very sensitive to climate change and observing them is definitely a good way to understand the changes," he said.

But he stressed that the individual sightings were not proof that their ecological habits had been effected by climate change, and that it was possible the birds could have been blown astray by a typhoon.

Climate experts claim Hong Kong could no longer have winters by the middle of this century with average temperatures rising by at least one degree Celsius in the next 20 years.