At the end of each month, Townshend takes students from Beijing Forestry University to the Wenyu River to record biodiversity. Over three and a half years, these surveys have created a living map of the area's seasonal changes. In his view, the country's environmental journey has been one of rapid changes. "When I first arrived, air pollution was obvious. Environmental laws were weak. Now, the direction is positive," he says.

Encouraged by the rise of local NGOs and youth involvement, Townshend is happy that many young people are becoming passionate about environmental protection. Still, challenges remain: overengineering of green spaces, habitat fragmentation, and a need for more scientific policies. But he is optimistic."We're learning," he says. "Beijing, with its migration spectacle, is a perfect place to teach the world about shared ecological responsibility."