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[00:01.451]They're cave swiftlets.
[00:02.451]Like bats they use echolocation to navigate.
[00:06.451]We need lights to see what's going on,
[00:09.452]but in the pitch black the swiftlets manage unerringly to locate their individual nesting sites,
[00:15.967]which are only a few centimetres across.
[00:17.958]It's a remarkable skill and one we still do not fully understand.
[00:23.955]These birds are unusual for another reason.
[00:27.959]Their little cup-like nests are made entirely from threads of saliva.
[00:33.709]It takes more than 30 days to complete one.
[00:39.199]Their nests are very precious objects, and not only for the birds.
[00:42.959]For 500 years people have been harvesting the nests of cave swiftlets.
