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The oilbird ( Steatornis caripensis), locally known as the guácharo, is a bird species found in the northern areas of South America including the Caribbean island of Trinidad. It is the only living species in the genus Steatornis, the family Steatornithidae, and the order Steatornithiformes. Nesting in colonies in caves, oilbirds are nocturnal feeders on the fruits of the oil palm and tropical Lauraceae. They are the only nocturnal flying fruit-eating birds in the world (the kākāpō, also nocturnal, is flightless). They forage at night, with specially adapted eyesight. However, they navigate by echolocation in the same way as , one of the few birds to do so. They produce a high-pitched clicking sound of around 2 kHz that is audible to humans.Snow (2008), pp. 137–143.
The specific name caripensis means 'of Caripe', and the generic name Steatornis means 'fat bird', in reference to the fatness of the chicks. The oilbird is called a guácharo or tayo in Spanish language, 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.
The fossil record of the family suggests that they were once more widely distributed around the globe. The first fossil oilbird was described by Storrs Olson in 1987 from a fossil found in the Green River Formation in Wyoming. The species, Prefica nivea, was probably not adapted to hovering flight or living in caves, unlike the oilbird. Some of the same families and genera of plants the present day oilbird feeds on have been found in the Green River Formation, suggesting that prehistoric species may have eaten the same fruit and spread the same seeds. Another species from the Upper Eocene has been discovered in France.Thomas, B.T. (2017). Oilbird ( Steatornithidae). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/52261 on 25 February 2017).
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. For example, the wings have deep wingtip slotting, like New World vultures, to reduce the stalling speed, and the wings have a low aspect ratio and low wing-loading, all to make the oilbird capable of flying at low speeds.
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 (f-number of 1.07). The retina is dominated by , 1,000,000 rods per 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 , 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.
In addition to clicks used for echolocation, oilbirds also produce a variety of harsh screams while in their caves. Entering a cave with a light especially provokes these raucous calls; they also may be heard as the birds prepare to emerge from a cave at dusk.
One such colony in Ecuador held a population of a hundred birds in a canyon with ledges protected by vegetation. Some smaller caves and gorges are used only for roosting. While it was once thought that oilbirds always or nearly always roosted in caves, canyons or gullies, researchers placing GPS trackers on non-breeding birds found that they regularly roost in trees in the forest as well as in caves.
It is a seasonal bird migration across some of its range, moving from its breeding caves in search of fruit trees. It has occurred as a rare vagrant to Costa Rica, Panama and Aruba. The Guácharo Cave (Oilbird Cave), in the mountainous Caripe district of northern Monagas, Venezuela, is where Alexander von Humboldt first studied the species.
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.
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