The investigation of annual changes of the number of Anas formosa during the wintering season from 2004 to 2008 showed that there was the largest number of individuals (78,000) in 2008. The monthly changes of the number of individuals at Junam Wetlands from October to March demonstrated that there was the largest number of individuals in December. On the other hand, the size of wintering population decreased in January. In order to analyze the factors affecting the decreasing population of A. formosa, daily investigations of wintering A. formosa were conducted in 2008. The results indicated the decreasing population and migration of A. formosa were mainly caused by interference factors such as fishing from motorboats and ice punching on the ice.