A pair of Oilbirds (CC BY 2.0)

A pair of Oilbirds (CC BY 2.0)

Not very long ago there was a time when the nestlings of Oilbirds or guacharo (Steatornis caripensis) were being used as night lamps. American Indians had been in the habit of using these juvenile birds as candles. They used to gather guacharo nestlings and put string through their bodies while they were still alive. After that they were put in the sun to dry. Nestlings with strings passed through their bodies were used as candles. Not only that, birds’ oil was also used for the purpose of cooking.

Oilbirds see with their eyes, ears and nose

A species of nightjars, guacharos were discovered by Alexander Humboldt, a traveler and biologist, in 1799 near Caripito in Venezuela. No one seriously studied these birds for the next 150 years. It was started only when echolocation became a subject of scientific research, and a special team of researchers arrived in Venezuela from USA. In northern South America and in Trinidad these birds live and breed in caves, the underground galleries of their homes go down to a depth of up to 650 meters where no daylight ever penetrates.

During the course of study, under the laboratory conditions, it was found that the birds could fly around freely in complete darkness without colliding with a wire stretched across the room. However, all the time they emitted a series of high-pitched, non-ultrasonic clicks of around 2 kHz, which are audible to the human ears. They emit bursts of astonishingly rapid clicks (as many as 250 per second), which helps them echolocate in the complete darkness. Oilbirds also utter squawks and shrieks that suggest their Spanish name, guácharo (“wailer”). At night, they fly out to feed, hovering while they pluck fruit from trees.

The instruments indicated that the clicks were very short and the time interval equaling a thousandth fraction of a second. Sound propagates in air at a speed of 340 meters per second, i.e. 15-20 times faster than the speed of flying guacharos. Hence a sonic pulse or clicks collides with an object and reflect from it long before the bird reaches it, thereby warning it about the presence of obstacles in its path. The bird receives quick and exhaustive information on the upcoming sections of its route by increasing the rate of clicks.

Besides the ability to echolocate oilbirds are also an extreme example of flying birds that lead a low light-level lifestyle. They forage for fruits at night. Using standard microscopy techniques, Martin et al. (2004) studied the retinal makeup of oilbird eyes and used an ophthalmoscopic reflex technique to find out the parameters of the birds’ visual fields. The eyes are comparatively small (axial length 16.1 ± 0.2 mm) with a maximum pupil diameter of 9.0 ± 0.0 mm, attaining a light-gathering capacity that is the highest recorded in any bird (f-number 1.07). The comparatively large pupil helps the oilbird eye gather four times more light than the human eye. The light-sensitive rod cells within the eye are stacked three deep, with a density of about 1,000,000 per mm2 – more than double the number usually found in vertebrate eyes. The tiered strategy, which has earlier been found only in deep-sea fish, maximizes the chance that every photon of light entering the eye will be intercepted. But extreme sensitivity comes at a cost. The retina’s tiered makeup makes it complicated for the brain to work out precisely where the light has come from, so oilbirds have a poor eye for details. It is therefore suggests that these nocturnal birds rely on a combination of information from smell and echolocation, as well as from sight to forage successfully.

The only nocturnal flying fruit-eating birds

However, the nightjars and their relatives are insectivores; the oilbird is a specialist fructivore (fruit-eating). It is also the only nocturnal flying fruit-eating birds in the world (the kakapo of New Zealand is flightless), oilbirds are so named, as they have a sort of oil in their bodies, which comes from the oil palm fruits on which they feed. Since the oil palm plantations are not very common, they have to fly 70-80 km to find food. While feeding, they cannot perch on branches like other birds because of their short legs. They grab food in flight, hovering in the air against the fruit clusters, in the fashion hummingbird do.

          They lay two to four white eggs on a pad of organic matter on a ledge high up in a cave. The young that may remain in the nest for 120 days, are fed by regurgitation. Nestlings of guacharo, relatives of common goatsuckers (so named because in the olden times, it was believed that these night-flying birds suck the milk of goats) of Europe are, normally, heavier than their parents. The weight of their two-month old nestling, with no plumage, is twice that of their parents, who have a wingspan of about one meter. It is not due to muscles; a young guacharo is like a bag of fat, which is why it is also called fat goatsucker, or simply fatties.

          Classified under monotypic family Steatornithidae, oilbirds are found in Southern America, from Trinidad and Guyana to Bolivia, it is a chestnut-coloured bird, with white spots. About 30 cm (12 inches) long, with a fanlike tail and long broad wings, large dark eyes, short legs, they live in caves and nest colonially on cave ledges and feed on fruits. Nocturnal by habits they have a short, hooked bill, a wide gape and rictal bristles.

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.


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