Oilbird
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 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 laurels. They are the only nocturnal flying fruit-eating birds in the world (the kakapo, also nocturnal, is flightless). They forage at night, with specially adapted eyesight. However, they navigate by echolocation in the same way as bats, one of the few birds to do so. They produce a high-pitched clicking sound of around 2 kHz that is audible to humans.[2]
Oilbirds are related to the nightjars and have sometimes been 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. Some research indicates that it should even be considered a distinct order (Steatornithiformes).[3]
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, 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.[4]
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.[5] 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.[6]
This is a large, slim bird at 40–49 cm (16–19 in), with a wing span of 95 cm (37 in). It has a flattened, powerfully hooked, beak surrounded by deep chestnut rictal bristles up to 5 centimetres (2.0 in) 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.[7] 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 oilbird is mainly reddish-brown with white spots on the nape and wings. Lower parts are cinnamon-buff with white diamond-shaped spots edged in black, these spots start small towards the throat and get larger towards the back. The stiff tail feathers are a rich brown spotted with white on either side.[6]
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.[6]
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