An endangered duck has been spotted in Hong Kong for the first time, thrilling birdwatchers who say its arrival may be due to the improving environment in the city’s reservoirs.

The scaly-sided merganser, known for its crest of wispy elongated feathers, was spotted in the central New Territories, Hong Kong Bird Watching Society on Monday said.

“Hong Kong is relatively safe for the duck because hunting is prohibited in the whole of the city,” said society director Yu Yat-tung. “The duck has come, possibly thanks to the improving environment in the city’s reservoirs. But more studies are needed to prove that.”

Local social media pages devoted to birdwatching have been awash with images and videos of the duck since mid-January, and dozens of aficionados have gathered with binoculars and cameras to capture the rare creature.

The society, which said it was the first time the scaly-sided merganser had been seen in Hong Kong since the society began its ornithological studies in 1957, declined to disclose the exact location of the bird for its protection.

Classified globally as endangered, the males and females exhibit different characteristics. The bird seen in Hong Kong, with a reddish brown crest and a grey back, appears to be a female.

The merganser stays in southeastern Siberia and northeastern mainland China for breeding, flying to the southern areas, Japan and Taiwan for winter, according to Birds of the World, a project undertaken by Cornell University in the United States.

Its population started declining in the 1960s and 1970s due to the loss of forests and freshwater habitats. The International Union for Conservation of Nature says fewer than 5,000 adults and adolescents remain, with most found in Russia.

Yu said he was pleased and excited to see the bird exploring new environments in Hong Kong, increasing the city’s biodiversity.

The ornithologist added that scaly-sided mergansers had been recorded in neighbouring Guangdong from time to time.

“Strangely, they never appeared in Hong Kong, considering how close the city is to Guangdong,” he said.

But he also did not rule out that it was a vagrant, meaning it could appear well outside its normal range by accident because of navigation errors or severe weather.

He also warned that the bird might head back to the north soon as temperatures continued to rise. Migratory birds usually depart Hong Kong in March.

The Bird Watching Society has recorded 36 kinds of duck in the city, including the whooper swan and five goose species.