A pair of whooper swans frolic at waterside of Gahai-Zecha National Nature Reserve, west China's Gansu Province, Feb. 17, 2011. Whooper swans, wild birds under the second class state protection, usually migrant to the wetland from Siberia for wintering in north China region. The Gahai-Zecha natural protection area, which is located in the Gannan Tibetan Autonomous Prefecture, attracts more than 300 whooper swans every winter. [Xinhua/Gao Jianjun]