Aerial photo taken on Nov. 28, 2019 shows swans swimming in the Hongjiannao Lake in Shenmu City, northwest China's Shaanxi Province. Hongjiannao Lake, located in Shenmu City, is a well-known desert freshwater lake in China. Photo:Xinhua

Aerial photo taken on Nov. 28, 2019 shows migrating birds flying over the Hongjiannao Lake in Shenmu City, northwest China's Shaanxi Province. Hongjiannao Lake, located in Shenmu City, is a well-known desert freshwater lake in China. Photo:Xinhua

Aerial photo taken on Nov. 28, 2019 shows swans swimming in the Hongjiannao Lake in Shenmu City, northwest China's Shaanxi Province. Hongjiannao Lake, located in Shenmu City, is a well-known desert freshwater lake in China. Photo:Xinhua

Aerial photo taken on Nov. 28, 2019 shows swans swimming in the Hongjiannao Lake in Shenmu City, northwest China's Shaanxi Province. Hongjiannao Lake, located in Shenmu City, is a well-known desert freshwater lake in China. Photo:Xinhua

Aerial photo taken on Nov. 28, 2019 shows a view of the frozen part of the Hongjiannao Lake in Shenmu City, northwest China's Shaanxi Province. Hongjiannao Lake, located in Shenmu City, is a well-known desert freshwater lake in China. Photo:Xinhua

Aerial photo taken on Nov. 28, 2019 shows swans strolling on the frozen part of the Hongjiannao Lake in Shenmu City, northwest China's Shaanxi Province. Hongjiannao Lake, located in Shenmu City, is a well-known desert freshwater lake in China. Photo:Xinhua

Aerial photo taken on Nov. 28, 2019 shows swans flying over the Hongjiannao Lake in Shenmu City, northwest China's Shaanxi Province. Hongjiannao Lake, located in Shenmu City, is a well-known desert freshwater lake in China. Photo:Xinhua

Aerial photo taken on Nov. 28, 2019 shows a winter view of the Hongjiannao Lake in Shenmu City, northwest China's Shaanxi Province. Hongjiannao Lake, located in Shenmu City, is a well-known desert freshwater lake in China. Photo:Xinhua