A refuge for birds is now a place for people, too.
Over the past two decades, the city has evolved from a green-roof backwater to a leader in the field.
Collisions led to 1,000 deaths a year in a single two-block radius, one study found.
Planners didn't want it to feel "over-horticultured" like NYC's High Line.
Urban evolution happens faster than anything Darwin saw.
The Arctic birds search for lemmings while dodging planes and traffic.