PLANET EARTH Caves

They're cave swiftlets. Like bats they use echolocation（回声定位） to navigate. We need lights to see what's going on, but in the pitch black the swiftlets manage unerringly（正确地） to locate their individual nesting sites, which are only a few centimeters 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（唾液）. It takes more than 30 days to complete one. The nests are very precious objects, and not only for the birds. For 500 years people have been harvesting the nests of cave swiftlets（金丝燕）. It's a very risky business.



With virtually no safety equipment and using ladders made from forest vines（藤） the gatherers climb into the highest reaches of the cave often more than 60 meters from the floor. The work may be hazardous（有危险的） in the extreme, but the rewards are great. The pure white nests of cave swiftlets are the main ingredient of birds' nest soup and gram for gram are worth as much as silver.