they are most vulnerable at the scheduled time of departure. 它们在出发阶段最容易遭受攻击
they have become important shelters for many species. 它们也成了许多动物的重要庇护所
which they use as a refuge from predators. 这里是它们躲避天敌的地方
but they could never have known that these flooded passageways were actually the beginning of subterranean rivers, 但他们从不知道这些水下通道 其实是地下河的入口
500 years ago they supported one of the world's great civilisations 500年前 它们维系着 一个世界上最伟大的文明
Like bats they use echolocation to navigate. 和蝙蝠一样 它们也利用回声定位来导航
but they must still survive the attacks of other, more specialised, birds of prey. 但它们仍须躲过 其它更专业的猛禽的攻击
They can stay down for more than 30 seconds 它们能在水下呆30多秒