Oilbirds

(Steatornithidae)

Class Aves

Order Caprimulgiformes

Suborder Steatornithes

Family Steatornithidae

Thumbnail description
Medium-sized birds, rather like nightjars but with a sharply hooked bill; nocturnal, feed only on fruit; breed and roost gregariously in caves; navigate by echolocation; long development period, fatty young

Size
17–19 in (43–49 cm); 0.8–1 lb (375–455 g)

Number of genera, species
1 genus; 1 species

Habitat
Low and montane primary forests mainly along mountains; require existence of large caves or gorges for roosting and breeding

Conservation status
Not threatened

Distribution
Largely restricted to South America, from Guyana, Trinidad, and Venezuela in the north, south along the Andes of Colombia, Ecuador, Peru, and Bolivia; northern Brazil, near Venezuelan border

Evolution and systematics

The Steatornithidae is a monotypic family found only in South America. The family's sole member, the oilbird (Steatornis caripensis Humboldt, 1817), is unique among members of the order Caprimulgiformes because it feeds only on fruit, nests and roosts colonially in caves, and echolocates. Nevertheless, oilbirds are recognizably nightjar-like, and their anatomy and the structure of their egg albumin and DNA confirm their phylogenetic affinity to other species of the order. Researcher David Snow proposed that oilbirds evolved from nocturnal or crepuscular insectivorous ancestors that benefited from social feeding because of the patchy distribution of fruiting forest trees. He theorized that because nestling fruit-eating birds develop slowly, predation pressure led to seeking safer, deeper recesses; the perfection of echolocation allowed the birds to occupy the deepest caves.

Reconstruction of such a sequence of events is unlikely because the fossil record of the family is fragmentary. However, two nearly complete fossil skeletons of early Steatornithidae from the early Eocene Green River Formation of Wyoming in the United States provide important evidence of the evolutionary origins of oilbirds. These extinct oilbirds (Prefica nivea) lived about 50 mya when the climate of western North America was subtropical and its landscape was similar to that of present day Neotropical savanna-woodland. The evidence suggests that the transition to fruit eating had taken place or was in the process of evolving by the early Eocene and that the lineage leading to Steatornis has been feeding on essentially the same specialized fruit diet for nearly 50 million years. In agreement with Snow's scenario, P. nivea was not yet adapted to cave life.

Oilbirds seem to be the extreme product of a limited offshoot that arose near the base of the caprimulgiform radiation. At the family level, the Steatonithidae should be regarded as a relict in South America rather than as an autochthonous endemic (a native that originated there).

Mitochondrial DNA analysis indicates that there is reduced genetic diversity among oilbird populations in Venezuela, possibly because the species has gone through a bottleneck. Alexander von Humboldt described oilbirds after

a visit, in 1799, to the now famous Cueva del Guácharo, near Caripe in northeastern Venezuela (hence its Latin name, caripensis). In Spanish, the common name for oilbirds is Guácharo.

Physical characteristics

Oilbirds have a hawk-like head with a strongly hooked bill but the body resembles that of a nightjar. Total length is 17–19 in (43–49 cm) and the wingspan is about 37.5 in (95 cm). Adult body mass is 0.8–1 lb (375–455 g). The plumage is rusty brown speckled with white dots. The wings are long, broad, and highly slotted and the wing-load is low, which allows birds to fly slowly in caves and to carry large loads. The tail is long and faintly barred black. At rest, the tail feathers form a tent-like inverted V, with the central feathers held higher and the outer feathers held lower. The legs are short and placed far forward so that at rest the head is held lower than the tail. As in other Caprimulgiformes, vibrissae (bristles on each side of the bill) are well developed. Plumage dimorphism is subtle, but males are slightly larger than females. Oilbirds do not cling to rock faces.

Distribution

Oilbirds are largely restricted to South America. They are locally distributed from Guyana, Trinidad, and Venezuela in the north, and along the Andes south through Colombia, Ecuador, Peru, and Bolivia. Oilbirds also inhabit northern Brazil on table-top tepuy mountains near the Venezuelan border. Wandering individuals have been recorded in Central America and on some Caribbean islands.

Habitat

Oilbirds inhabit evergreen lowland and montane forests. They require large expanses of forest where ripe fruit is available year round and nearby caves or gorges where they can nest and roost. They have an exceptionally broad altitudinal range that extends from sea caves to over 10,500 ft (3,200 m) in the Andes.

Behavior

Oilbirds are the only nocturnal fruit-eating birds in the world. They roost and nest colonially in caves in which they navigate by echolocation. Oilbirds are strongly gregarious, and thousands of birds can inhabit a single cave. Some of the largest colonies in Venezuela may each have an estimated 10,000 birds.

Oilbirds spend the day sitting quietly on their nests or perched on ledges of a rock wall. Shortly before dusk they awaken and become noisy and restless until the darkness of night settles over the forest. Then they leave the caves to feed, and they seem to forage in groups.

Oilbirds can navigate in total darkness by echolocation. They utter echolocating clicks with a bilaterally asymmetrical bronchial (not tracheobronchial) syrinx (or vocal organ). Most of the acoustic energy of clicks lies between 1 and 15 kHz, which is almost entirely within the audible human range. Clicks typically last 40 to 80 ms and are produced at a rate of up to 10–12 per second. The frequency of clicks depends on a bird's proximity to obstacles, with the frequency increasing as the bird approaches the obstacle. Because oilbirds use low frequencies for echolocation, their sonar system is crude compared to those of many insectivorous bats. Among birds, oilbirds' ability to echolocate is shared only with some paleotropical swiftlets. In addition to echolocating clicks, oilbirds emit various harsh contact calls and high-intensity agonistic squawk-like vocalizations.

Feeding ecology and diet

Oilbirds are entirely frugivorous (fruit eating); adults and nestlings feed on fruit pulp only. They feed almost exclusively on the fruits of trees belonging to three plant families: Lauraceae (the avocado family), Arecaceae (palms), and Burseraceae. Oilbirds do not use their clicking echolocating call when foraging; instead, they seem to rely on their keen nocturnal vision and possibly on their sense of smell.

Oilbirds grab fruits on the wing and swallow them whole. After stripping the nutritious fleshy pulp they regurgitate seeds intact. Fruits are also fed whole to nestlings. The pulp of most fruit consumed by oilbirds is rich in fats and consequently

in energy. The Lauraceae eaten by oilbirds in Venezuela average about 50% fat and 32 kJ per dry gram. Such a high-energy food allows oilbirds to forage over vast expanses of forest. During the nonbreeding season when most laurel fruit becomes scarce, birds fitted with radio harnesses in Venezuela have been recorded foraging over 70 mi (110 km) away from their daytime roosting cave. They also migrate seasonally between caves.

Reproductive biology

Oilbirds spend most of the day in pairs, even when they are not nesting; apparently they are monogamous and form permanent pair bonds. Pairs fiercely defend their nest but tolerate other established pairs even within bill distance.

Oilbirds typically build their nest on ledges 33–66 ft (10–20m) above the cave floor, although researchers have seen them nesting on the ground in Andean caves. The nest is a cylindrical mound about 16 in (40 cm) in diameter made of regurgitated plant fiber compacted into a firm paste. Two to four white eggs are laid at intervals of two to five days. Both adults share in incubation, brooding, and feeding of the young. Incubation lasts 32–35 days and begins when the first egg is laid.

Newly hatched chicks (0.4–0.5 oz; 12–15.5 g) bear an egg tooth, are pinkish, are almost completely naked, and their eyes are closed. Chicks grow very slowly, leaving the nest when they are between 110 and 120 days old. By day 70, nestlings have an appearance similar to that of adults, except that the tail and wings are shorter. At this point they reach a peak weight of about 1.3 lb (600 g), nearly 50% above average adult weight (14.5 oz; 410 g). Afterwards, they are fed at a slower rate and lose weight until reaching adult mass at fledging. Such an unusual pattern of weight change seems to be related to oilbirds' frugivorous habits. Fruits eaten by oilbirds have a low protein-to-energy ratio, and it is presumed that to meet their protein requirements, nestlings must ingest an excess of energy that is temporarily stored as fat. The diet's low protein content might also account for oildbirds' slow rate of development.

Breeding is highly seasonal, largely coinciding with the rainy season, but the onset of breeding varies from place to place. In the Caripe Cave most egg laying occurs in the second half of April and May, which coincides with the fruiting peak of Lauraceae. In nearby Trinidad, breeding is less seasonal; a number of birds are likely to be found in several breeding stages at any time of the year.

Conservation status

Oilbird colonies are widespread but patchily distributed. Threats include poaching of nests and forest destruction. The species is not threatened, but its specialized diet and very large home range make it vulnerable to deforestation. Several colonies disappeared in Trinidad and Venezuela during the last decades of the twentieth century, but all of them were small. The species receives ample legal protection in several parks and conservation areas throughout South America.

Significance to humans

Oilbird nestlings are a rich source of energy and protein. At one time, throughout their range, native South Americans ate oilbird nestlings and used their fat for lamp oil. At the turn of the twenty-first century, oilbirds are legally protected in most countries where they are found.

Oilbirds are unique among birds and large oilbird colonies are one of the great ornithological spectacles of the world. Easily accessible oilbird caves such as the one in Caripe are important tourist attractions. Oilbirds have not successfully been kept in zoos, but at least one individual is known to have been kept as a pet for years.

Resources

Books

del Hoyo, J., A. Elliot, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 5. Barn-Owls to Hummingbirds. Barcelona: Lynx Edicions, 1999.

Roca, R.L. Oilbirds of Venezuela: Ecology and Conservation. Cambridge, MA: Publications of the Nuttall Ornithological Club, No 24, 1994.

Suthers, R.A., and D.H. Hector. "Individual Variation in Vocal Tract Resonance May Assist Oilbirds in Recognizing Echoes of Their Own Sonar Clicks." In Animal Sonar, edited by P.E. Nachtigall and P.W.B. Moore. New York: Plenum Press, 1986.

Periodicals

Bosque, C., R. Ramírez, and D. Rodríguez. "The Diet of the Oilbird in Venezuela." Ornitologia Neotropical 6 (1995): 67–80.

Gutierrez, P.C. "Mitochondrial-DNA Polymorphism in the Oilbird (Steatornis caripensis) in Venezuela." Auk 111 (1994): 573–578.

Konishi, M., and E.I. Knudsen. "The Oilbird: Hearing and Echolocation." Science 204 (1979): 425–427 Snow, D.W. "The Natural History of the Oilbird, Steatornis caripensis, in Trinidad, W. I. Part 1 & 2." Zoologica 46 (1961): 27–48 and 47 (1962): 199–220.

Suthers, R.A., and D.H. Hector. "The Physiology of Vocalization by the Echolocating Oilbird, Steatornis caripensis." Journal of Comparative Physiology (A) 156 (1985): 243–266.

Tannenbaum, B., and P. Wrege. "Ecology of the Guácharo (Steatornis caripensis) in Venezuela." Boletín de la Academia de Ciencias Físicas Matemáticas y Naturales 38 (1978): 83–90.

Tannenbaum, B., and P. Wrege. "Breeding Synchrony and Nestling Mortality in Oilbirds Breeding in the Cueva del Guácharo." Boletín de la Sociedad Venezolana de Ciencias Naturales 142 (1984): 121–137.

Carlos Bosque, PhD