#guacharo

Oilbirds are related to the nightjars and usually placed with these in the order Caprimulgiformes. However, the nightjars and their relatives are insectivores while the oilbird is a specialist fructivore, and it is sufficiently distinctive to be placed in a family (Steatornithidae) and suborder (Steatornithes) of its own. The caripensis of the binomial name means "of Caripe", and Steatornis means "fat bird", in reference to the fatness of the chicks. The oilbird is called a guácharo or tayo in Spanish, both terms being of indigenous origin. In Trinidad it was sometimes called diablotin (French for "little devil"), presumably referring to its loud cries, which have been likened to those of tortured men. The common name "oilbird" comes from the fact that in the past chicks were captured and boiled down in order to make oil.

This is a large, slim bird at 16–19 inches, with a wing span of 37 inches. It has a flattened, powerfully hooked, bill surrounded by deep chestnut rictal bristles up to 2 inches long. The adult weighs 350–475 g (12.3–16.8 oz) but the chicks can weigh considerably more, at up to 600 grams (21 oz), when their parents feed them a good deal of fruit before they fly. The feathers of the oilbird are soft like those of many nightbirds, but not as soft as those of owls or nightjars, as they do not need to be silent like predatory species. The feet are small and almost useless, other than for clinging to vertical surfaces. The long wings have evolved to make it capable of hovering and twisting flight, which enables it to navigate through restricted areas of its caves. 

The eyes of oilbirds are highly adapted to nocturnal foraging. The eyes are small, but the pupils are relatively large, allowing the highest light-gathering capacity of any bird. The retina is dominated by rod cells, 1,000,000 per rods mm2, the highest density of any vertebrate eye, which are organised in layers, an arrangement unique among birds but shared by deep-sea fish. They have low numbers of cone cells, and the whole arrangement would allow them to capture more light in low light conditions but probably have poor vision in daylight. Although they have specially adapted vision to forage by sight, they are among the few birds known to supplement sight by echolocation in sufficiently poor light conditions, using a series of sharp audible clicks for this purpose. The only other birds known to do this are some species of swift.

Oilbirds are nocturnal. During the day the birds rest on cave ledges and leave at night to find fruit outside the cave. It was once thought that oilbirds only roosted in caves, and indeed never saw daylight, but studies using GPS/acceleration loggers found that non-breeding birds only roosted in caves or other rock shelters one night in three, the other nights roosting in trees. The scientists responsible for the discovery also found that birds roosting in caves were highly active through the night, whereas birds roosting in the forest were far less active. They hypothesised that each environment carried costs; birds roosting in the forest were more vulnerable to predators and birds roosting in caves expended considerable energy competing with rivals and defending nesting and roosting ledges.