The number of endangered black-faced spoonbills returning to Hong Kong and Shenzhen for the winter fell almost 30 per cent this year.
The decline put the population of the large, white, water-migratory birds calling Deep Bay a winter home at a 10-year low, the Bird Watching Society said.
Just 252 birds were recorded, down from 351 last year. Their numbers on the mainland also dropped to 339 from 363.
One-fifth of the global spoonbill population used to flock to Deep Bay, sandwiched between Hong Kong and Shenzhen, as a key wintering site a few years ago. This year, only 9 per cent came, a society report said.
Rapid development along the coastal mainland might have disrupted migratory patterns, though a clear reason for the steep decline could not be pinpointed, the society's research manager, Yu Yat-tung, said.
"The birds must fly long distances and this requires lots of energy," Yu said.
"If they don't get proper rest or adequate food on the way, they usually don't make it."
In particular, spoonbill numbers in Fuqing , Fujian province, dropped about 60 per cent, Yu said, pointing to the construction of an industrial park near a key habitat.
He speculated this might have affected the number of birds arriving in Hong Kong.
The society conducted the study with organisations from 15 other locations in east and southeast Asia - the only known habitats of the rare bird.
Around October, the spoonbills fly south from their breeding grounds in the Korean Peninsula to the warmer tropics. The Mai Po Nature Reserve near Yuen Long is a major wintering site.
Yu said a further decline was expected if the Nam Sang Wai wetlands were developed.
A plan by private developers to build flats on the wetlands was rejected last month.
The global population of spoonbills rose by one bird last year, from 2,725 to 2,726.
The Agriculture, Fisheries and Conservation Department said it had been monitoring the migratory birds for years. Key conservation efforts included providing the winter guests with food and intertidal ponds.