Guácharo Cave houses one of the largest known colonies of the guácharos or Oil Birds or Diablotin (Steatornis caripensis). They are of a dark grey-blue colour, grow up to a size of 60cm and a wingspan of 110cm, and look like a bird of prey. Unique among birds is the ability to navigate using high-pitched squawks for echo-location like bats.

The nocturnal birds leave the cave every evening after dark to feed on fresh fruit. The fruit and seeds they bring back into the cave continuously, are the base of a whole ecosystem of plants and animals, living in the cave. Many semen start to grow in the moist and fertile cave floor, but without sunlight their life is rather short. Among the animals feeding on the remains of the birds are wood-louses, crickets, millipedes, mice, fish, and crabs.

Alexander von Humboldt made the first scientific exploration of the cave. He observed the bats, the birds, and the other animals in the cave and described them in his book about this South America journey. He named the birds Fettschwälme, which is the base of the above names in other languages. As this book was sold many times all over Europe, a very early bestseller, the cave became rather famous. But because of its remote location, at this time like today, only a few people were able to visit the cave.

When Alexander von Humboldt visited the cave in 1799 the local monks described to him, how the local Indians hunted the birds for their fat deposits. At midsummer they entered the cave with long poles and destroyed most of the thousands of nests lining the high roof. The young birds were killed and stripped of the layer of fat between their thighs, which was melted in clay pots over fires and filtered to produce a pure, clear, odourless liquid. As superstitious dread prevented the Indians from entering very far into the cave, and then only on an annual basis, the colony managed to survive!

Alexander von Humboldt visited only the first 472m of the cave, as his native guides refused to proceed into the cave any further. But he saw the most interesting feature of the cave, the birds, and described this in his book. Because of his great influence a monument of Humboldt was erected opposite the cave, at a view which was painted by the travel companion of Humboldt.