CHONGQING, Nov. 7 (Xinhua) -- Nearly 2,000 black-headed gulls have arrived in southwest China's Chongqing Municipality on the upper reaches of the Yangtze River.

Ornithologists said the Chongqing section of China's longest river has seen the largest migratory flock in the past decade.

The gulls have mainly arrived at a rare-fish nature reserve on the Yangtze River, a fact that is likely related to the measures to protect the ecology and environment there, including fishing bans, said Xu Zhiqing, a researcher with the Chongqing Natural History Museum.

Xu said the main food of black-headed gulls is small fish and shrimps, and the protection and restoration of fishery resources on the Yangtze River provides them with more food.

Authorities have taken measures to guide citizens to watch the gulls in an orderly fashion. They also regularly disinfect the habitat and monitor the health condition of the wintering gulls.

Kuang Gaoxiang, an ornithologist with the Chongqing Zoo, said that the black-headed gulls will probably overwinter in Chongqing if there is sufficient food, effective protection and minimal human interference.