Many people set off fireworks to mark the arrival of the new year, so the holiday is a time when they have to stay vigilant against fire hazards, Tian said.

The couple has also welcomed some unexpected winged visitors since the turn of the century.

In the spring of 2000, several egrets and herons came to the mountains and took advantage of a nearby reservoir, and they didn't leave until the weather turned cold.

Egrets and herons are often recognized as environmental monitors, as they have very strict requirements for environmental conditions in their habitats.

More birds have come in the ensuing years. Currently, the number of birds that inhabit the two mountains during the warm period every year stands at around 2,000.

Last year, local authorities named the mountains "Luling" — which literally means "the mountains herons inhabit" — as tourists kept pouring in, lured by the trees and rare birds.

The couple has no plans to quit their work, and they hope their children will follow in their footsteps.

"We cherish the mountains and trees as we cherish our own children," Lu said. "Even as we grow old, they will continue to guard the woods and watch over the birds."