The Hong Kong Bird Watching Society warned on Sunday it is worrying that the number of sparrows in the city has decreased by around 30 percent in the past five years.
The society said its surveyors found around 211,100 Eurasian Tree Sparrows in the SAR in May – the lowest figure since 305,000 were spotted in their first annual survey in 2016.
Pang Chun-chiu, an ornithologist with the organisation, said further studies need to be carried out to discover why there has been a drastic decline, but a lack of nesting space and food shortages could be to blame.
"They nest in crevices which used to be abundant in a modern city like Hong Kong... [so] the glass [facades] of the buildings might reduce the nesting spaces for sparrows," he said.
Pang explained the decline of the sparrow population might also reflect a worsening urban environment in Hong Kong.
"Sparrows are a tough species, actually. They adapt to changes very quickly, and they eat a wide range of food – from plant seeds, insects to human leftover food. There might be something getting worse here which is affecting the sparrow population."
He called on people to help protect the birds, adding that residential areas are their favourite spots.
For example, he said people should not disturb sparrows if they see them making nests in buildings during their breeding season between March and June.