Weak-legged house swifts unable to keep their feet on the ground

'I've heard there's a kind of bird without legs that can only fly and fly, and sleep in the wind when it is tired. The bird only lands once in its life ... that's when it dies,' goes the memorable quote from Leslie Cheung in the film Days of Being Wild.

And in reality, such a bird exists and lives under the roof of Chinese University. The house swift, whose legs are too weak to support its body weight, never lands on flat ground during its lifetime.

House swifts are small and they fly at great speeds. They are about 15cm long and live under roofs in Hong Kong. There are more than 900 house swifts in the city, according to conservation consultant Tsim Siu-tai.

House swifts' bodies have evolved to live on cliffs by the sea, he said. Their legs can hook them firmly to rocks.

'Either they fly or they return to their nests,' said Hong Kong Bird Watching Society project manager Lo Wai-yan. 'They stay in the air when they catch insects to eat. They are used to building nests around human homes, which is similar to what swallows do.'

The birds could be found in old houses in Tuen Mun and Sheung Shui.