The number of the endangered black-faced spoonbills in Hong Kong and Shenzhen has dropped for three consecutive years, although its global population is on the rise, a survey has found.
The study, conducted at 15 locations throughout Asia by the city's Bird Watching Society, recorded 2,725 of the rare migratory birds over three days in January.
This is the highest figure of the spoonbills recorded since the study began a decade ago.
The number of the large, white water birds has been increasing steadily since, from the initial 351 seen in 1993.
But society's volunteers also found that the number of spoonbills recorded in Deep Bay, between the city and neighbouring Shenzhen, had dropped more than 10 per cent to 351 this year from last year's 393. The figure was 411 in 2011.
While the reason behind the birds' decline in the two cities was unclear, society research manager Yu Yat-tung said it could be linked to warmer weather on the mainland as a result of climate change.
"The birds may have found places north to Hong Kong warm enough and chosen to stay there, nearer to their breeding grounds," Yu said.
The number of black-faced spoonbills found on the mainland was 363 this year, 35 more than that of last year's.
Other places in which the birds were spotted this year included Japan, Vietnam, Macau, South Korea, the Philippines and Thailand.
Yu said although the city's government was doing well in conserving the Mai Po nature reserve, future development at the Nam Sang Wai wetlands nearby could harm the spoonbills.
He was also concerned about the impact of the mainland's fast pace of development on the birds, as well as illegal hunting.
Although their global population was on the rise, the black-faced spoonbills were still in danger of extinction, Yu said.
While the increase in their numbers could be a result of better conservation, he said, it could also be partly attributed to a rise in the number of volunteers conducting the census.