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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.
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