The Gomantong limestone caves, once accessible only by a stream off the Kinabatangan River, have been harvested for the valuable bird's nests for centuries. The harvesting of bird's nests is today regulated by the Wildlife Department.Two cave complexes produce different types of nests, the less valuable "black" nests which feathers, gathered in the easily accessible Simud Hitam Cave. This cave, with its roof soaring up to 90 metres high, is just a 5 minutes walk from the Registration Centre and picnic area, and gives visitors a glimpse of the remarkable life within a limestone cave. Swiftlets, who make the valuable nests, and bats share the caves with thousands of insects which live in the rich guano on the floor. The Simud Puteh Cave complex, much larger and less easily accessible than Simud Hitam, is where the more valuable "white" nests made of pure saliva are found.
Twice a year, licensed collectors gather the nests in a dangerous operation involving the use of rattan ladders, ropes and poles. The nests are first havested just after the birds have made them (between February and April). The birds then build new nests, which are left undisturbed until after the aggs have been laid and hatched; these nests are then gathered, some time between July and September