They're cave swiftlets. Like bats they use echolocation to navigate. We need lights to see what's going on, but in the pitch black the swiftlets manage unerringly to locate their individual nesting sites, which are only a few centimetres across. It's a remarkable skill and one we still do not fully understand. These birds are unusual for another reason. Their little cup-like nests are made entirely from threads of saliva. It takes more than 30 days to complete one. Their nests are very precious objects, and not only for the birds. For 500 years people have been harvesting the nests of cave swiftlets.


它们是穴金丝燕。和蝙蝠一样，它们依靠回声来做导向。我们需要有光线才能看清楚东西，但是，在一片漆黑的情形下，穴金丝燕却可以准确找到只有几厘米宽的巢穴。这是一种非凡的本领，到现在人类还无法完全了解。这种鸟还有一种不同寻常之处。它们像杯子一样的巢完全是用自己的唾液筑成的。要筑成这样一个巢穴至少需要整整30天时间，它们的巢非常珍贵，这不仅对穴金丝燕而言。500年来，人类一直在摘取这些金丝燕的窝。