Swans are seen at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province, Oct. 31, 2021. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Swans are seen at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province, Oct. 31, 2021. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Swans are seen at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province, Oct. 30, 2021. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Swans are seen at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province, Oct. 31, 2021. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Swans are seen at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province, Oct. 31, 2021. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Swans are seen at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province, Oct. 31, 2021. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Aerial photo taken on Oct. 31, 2021 shows swans at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Swans are seen at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province, Oct. 31, 2021. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Swans are seen at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province, Oct. 30, 2021. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Swans are seen at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province, Oct. 31, 2021. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Swans are seen at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province, Oct. 30, 2021. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)

Aerial photo taken on Oct. 31, 2021 shows swans at the Hekou Reservoir wetland in Yulin, northwest China's Shaanxi Province. To the south of the Maowusu Desert, the Yuyang District has witnessed improvement in eco-environment thanks to decades of afforestation efforts. (Xinhua/Tao Ming)