The Hong Kong Bird Watching Society on Wednesday said the number of sparrows in the SAR has reached the highest level in five years, though they're still struggling to work out why.

The group said its surveyors found around 287,000 Eurasian Tree Sparrows in Hong Kong in May – a 36 percent increase on last year.

Its director, Yu Yat-tung, said there've been no drastic changes in weather data nor in the urban environment to explain the rebound.

However, he said sparrows thrive in older districts such as Sham Shui Po and Wong Tai Sin, adding that many nest around window air-conditioning units or external wall structures.

“From our observation, we can see that sparrows actually quite like areas that they can hide, such as the space of air-conditioners outside building walls,” he explained.

“For those new buildings, walls will be much more nicer, all things are on one surface. We have no holes, no areas for birds to stop. Sparrows cannot stay there.”

He called on people to continue protecting the birds and their nesting areas.