These birds are unusual for another reason.
这些鸟之所以特别还有另外一个原因

Question

but in the pitch black the swiftlets manage unerringly to locate their individual nesting sites,
但是金丝燕却能在黑暗中准确无误地到达自己的筑巢点
which are only a few centimetres across.
而它们的巢窝相隔不过几厘米远
It's a remarkable skill and one we still do not fully understand.
我们至今仍未能完全了解这种了不起的本领
These birds are unusual for another reason.
这些鸟之所以特别还有另外一个原因
Their little cup-like nests are made entirely from threads of saliva.
它们小巧的杯状燕窝完全由唾液丝筑建
It takes more than 30 days to complete one.
每个燕窝得花上30多天才能完工
The nests are very precious objects,
燕窝是非常珍贵的东西

来自《行星地球第一季第四集 Planet Earth S01E04 洞穴》第 118 行