Li, who founded the wildlife conservation service in 2000, said his team has recorded more than 300 kinds of migratory birds, of which the most seen include the whooper swan, the whistling swan, the mute swan and various kinds of wild geese and ducks.

The Black Leopard workers usually record about 30,000 to 40,000 migratory birds stopping in the reservoir every spring, but the number this spring has exceeded 60,000, Li said.

That is because the birds, during their travel southward, left Beijing early last year, as they found it too cold. So they spent more time in the warmer south before heading north later, Li said.

"They have enjoyed a very good rest in the south. So this spring we saw them (in Beijing) with stronger physique, better vigor and brighter feather," Li added.

Migratory birds recording is a small part of Black Leopard's very busy workload. The civil organization's work covers the mountainous suburbs between Hebei and northwest and southwest Beijing, which are rich in wildlife.

It has set up about 110 infrared cameras there and dispatches its members and volunteers to patrol the regions to survey biodiversity, guard against poaching and maintain the cameras.

Such patrols can be dangerous because during their hikes, usually of between eight and 15 kilometers, the workers sometimes have to walk on riverbeds, exposed to the risks of floods to get to their destinations, Li said.

But their effort is well recompensed.