A 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 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 laurels. 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 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.
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. 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.
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 rod cells, 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 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.
The Guácharo Cave was Venezuela's first national monument and is the centerpiece of a national park; according to some estimates there may be 15,000 or more birds living there. Colombia also has a national park named after its "Cueva de los Guácharos", near the southern border with Ecuador. Oilbirds have been reported in various other places along the Andean mountain chain, including near Ecuador's Cueva de los Tayos and in Brazil: they are known to dwell as far south as the Carrasco National Park in Bolivia. Dunston Cave, at the Asa Wright Nature Centre in Trinidad, is home to about 200 nesting pairs. The species is classified as 'Least Concern' by the IUCN red list as of October 2016, despite a decreasing population.