Bays and lakes in Rongcheng, Shandong province, are seeing more hustle and bustle than usual as thousands of whooper swans prepare for their journey to Siberia as the weather warms.

"Flapping wings, calling and flying back and forth, the whooper swans used to be quiet, but these days they've become restless," said Liu Zhibin, who has been working at the Rongcheng Swan National Nature Reserve for five years.

The 68-year-old and his wife patrol the reserve twice a day to take care of the swans, including checking on their numbers and whether their living conditions are damaged.

"Some whooper swans have shown signs that they are preparing to set out on their journeys, and most will start migrations in early March," he said.

The 16.7-square-kilometer reserve has several rivers, which makes the site rich in habitat diversity, including tidal flats, an estuary and sandbar lagoon.

Its wetland habitats and abundant aquatic plants and fish are important to migratory water birds, with over 20,000 estimated to visit the reserve each year, according to reserve management.

Thousands of whooper swans migrate annually, including from the Siberian region, and stay in Rongcheng for around five months, making the city one of the largest wintering grounds for whooper swans in the world.

"A sound living environment is attracting more and more swans to spend the winter in Rongcheng," Liu said.

"Changes in the environment are obvious. Ten years ago, sewage and garbage were discharged into lakes and bays. But now, everywhere is clean."

The local government launched a campaign in 2015 to improve sewage treatment and river water quality amid China's efforts to emphasize sound ecological development as a basis for sustainable social and economic development.