Wild Duck Lake National Wetland Park in suburban Beijing's Yanqing district is an essential stopover for migrating birds on the East Asia-Australasian Flyway. [Photo provided to China Daily]

It closes from mid-November to mid-March to protect more than 100,000 migrating birds that swoop in during the period, including whooper swans and gray cranes.

(Professional bird-watchers and conservation organizations can apply for special admission during this period but must follow guidelines.)

It's the busiest time of the year for Fang and his colleagues, who spend most of their days patrolling in the cold.

The park hosts 80 monitoring sites and four signal-transfer stations, so images can be transmitted in real time. Staffers also use unmanned aerial vehicles, plus remote sensing, geographical information systems and global positioning systems.