Oilbird

Oilbird
Conservation status
Least Concern (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
(unranked):	Cypselomorphae
Order:	see text
Suborder:	Steatornithes
Family:	Steatornithidae Bonaparte, 1842
Genus:	Steatornis Humboldt, 1814
Species:	S. caripensis
Binomial name
Steatornis caripensis Humboldt, 1817

The oilbirds (Steatornis caripensis), locally known as the guácharo, are birds found in the northern areas of South America (including the island of Trinidad in the Republic of Trinidad and Tobago). Nesting in colonies in caves, they 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 is flightless). They forage at night, with specially adapted eyesight. However they navigate by echolocation in the same way as bats, and are the only birds known to do so. They produce a high-pitched clicking sound of around 2 kHz that is audible to humans.

Description

This is a large, slim bird at 40–49 centimetres (16–19 in), with a wing span of 95 centimetres (37 in). It has a flattened, powerfully hooked, bill surrounded by deep chestnut rictal bristles up to 5 centimetres (2.0 in) long. The adult weighs 350–475 grams (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 oilbird is mainly reddish-brown with white spots on the nape and wings. Lower parts are cinnamon-buff spotted with white. 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 make it capable of hovering and twisting flight, which enables it to navigate through restricted areas of its caves.

Taxonomy and etymology

Oilbirds are related to the nightjars and usually 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 (which does not yet have a valid taxonomic name).

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. The common name "oilbird" comes from the fact that in the past chicks were captured and boiled down in order to make oil.

Distribution and habitat

The oilbird ranges from Guyana and the island of Trinidad to Venezuela, Colombia, Ecuador, Peru, Bolivia and Brazil in forests and woodlands with caves. It is a seasonal migrant, 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.

Behaviour

Oilbirds are nocturnal; during the day the birds rest on cave ledges and leave to find their food at night. Although they have specially adapted vision to forage by sight, they are the only birds known to navigate by echolocation in sufficiently poor light conditions, using a series of sharp audible clicks for this purpose. They 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.

Breeding

Oilbirds are colonial cave nesters. The nest is a heap of droppings, usually above water—either a stream or the sea—on which 2-4 glossy white eggs are laid which soon become stained brown. These are rounded but with a distinctly pointed smaller end and average 41.2 millimetres (1.62 in) by 33.2 millimetres (1.31 in). The squabs become very fat before fledging, weighing up to half again as much as the adult birds do.

Status and conservation

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.

References

• Herklots, G. A. C. (1961). The Birds of Trinidad and Tobago. Collins, London. Reprint 1965.
• Holland RA, Wikelski M, Kümmeth F, Bosque C, 2009 The Secret Life of Oilbirds: New Insights into the Movement Ecology of a Unique Avian Frugivore. PLoS ONE 4(12): e8264. doi:10.1371/journal.pone.0008264
• Stiles and Skutch, A guide to the birds of Costa Rica ISBN 0-8014-9600-4
• Snow, D.W. (2008). Birds in Our Life. William Sessions Limited. ISBN 978-1-85072-381-3 (pbk).

External links

• Cueva del Guácharo (ShowCaves)
• Oilbird videos, photos & sounds on the Internet Bird Collection
• The oilbird's visual acuity.
• Oilbird sounds on xeno-canto.org
• (Spanish) Caripe.net - La Puerta de Entrada
• Oilbird Caves of Trinidad Accessed 30 March 2011