英语听力：猛玛象进城记—18

This mass roosting takes place soon after the summer nesting season. It’s a chance for the birds to molt before they fly south for the winter. As Vaux’s swift numbers reach a peak in mid September, they attract attention from the locals. Coopers hawks and peregrines pluck a meal from the tumbling mass of birds. In as little as 20 minutes, as many as 40,000 swifts pack into the chimney for the night.

While a Portland chimney has become a substitute for a tree, another roosting creature has chosen a road bridge in Austin, Texas as its cave. Visitors to Austin have to wait till evening for a glimpse of what lurks within the bridge.

As the sun goes down, Mexican free-tailed bats begin a breath-taking commute out of the city. This exodus only takes place in summer. Free-tailed bats arrive in Texas for the spring, returning to Mexico for the winter. More than a million of them leave the city at dusk and head into surrounding countryside to feed on insects. The sky over Austin swarms with these aerial commuters.