Birds
Oilbird
Steatornis caripensis · also called oilbird, guácharo, diablotin
The oilbird is the world’s only night-flying fruit eater — a reddish-brown, hawk-billed bird that lives its daylight hours in the pitch dark of deep caves, in screaming colonies thousands strong, and pours out at dusk to hunt palm and laurel fruit by smell and echo.
Overview
Oilbirds live in northern South America and Trinidad, roosting and nesting on ledges deep inside caves and gorges — sometimes hundreds of metres into total darkness — and flying out at night to feed. They eat only fatty fruit — palms, laurels, incense trees — swallowed whole while hovering, and regurgitate the seeds, making them major long-distance seed dispersers.
Night senses
Uniquely among birds, the oilbird is nocturnal, flying, and frugivorous at once. It has the densest rod photoreceptors of any vertebrate eye, a strong sense of smell, and echolocation — audible ~2 kHz clicks that let it dodge cave walls in the dark, though too coarse to find food. It can fly 75–150 km round trip in a night to reach fruiting trees.
The name
Nestlings gorge on oily fruit and grow far fatter than adults. People once harvested the chicks and boiled them down for a clear cooking and lamp oil — hence “oilbird.” Humboldt described the famous Cueva del Guácharo in Venezuela in 1799.
Status
The oilbird is Least Concern but colonial, concentrated at a few cave sites, which leaves it exposed to disturbance, chick harvesting, and loss of nearby feeding forest.
Oilbird: common questions
What makes the oilbird unique among birds?
It is the only bird that is nocturnal, flies, and eats fruit — a combination found in no other species. Nocturnal birds like owls and nightjars are hunters of animal prey; fruit-eating birds like toucans, pigeons, and hornbills are active by day. The oilbird alone forages for fruit at night on the wing. To do it, it has assembled a toolkit no other bird has all of — large light-gathering eyes with the densest known concentration of rod photoreceptors of any vertebrate, echolocation for moving through dark caves, an unusually good sense of smell for finding aromatic fruit, long bristles and a wide gape for plucking and swallowing fruit whole in flight, and the ability to hover briefly like a giant nightjar.
How does the oilbird echolocate, and is it like a bat?
It works on the same principle — emit a sound, listen for the echo, judge distance from the delay — but it is much cruder than bat sonar. Oilbirds produce sharp clicking bursts at frequencies around 2 kilohertz, well within human hearing, whereas most bats use ultrasound far above it. The lower frequency and longer wavelength mean oilbird echolocation can detect obstacles down to a few centimetres across but cannot resolve fine detail or small insects. It is enough to keep a bird from flying into a cave wall or a neighbour in total darkness, but for finding food the oilbird switches to vision and smell once it is outside. Only a handful of birds echolocate at all — the oilbird and some cave swiftlets.
Why is it called an oilbird?
Because of its fat chicks. Oilbird nestlings are fed almost entirely on oily fruit pulp and become extremely fat before they fledge, at times weighing around half again as much as an adult bird. For centuries, people in Venezuela, Trinidad, and elsewhere climbed into the nesting caves once a year, collected the plump squabs, and rendered them down over a fire into a clear, semi-liquid, nearly odourless oil that keeps well and was used for cooking and to fuel torches and lamps. The Spanish name guácharo (roughly "one who cries or wails") refers to the bird's harsh screaming calls, and the famous type locality is the Cueva del Guácharo in Venezuela, visited and described by Alexander von Humboldt in 1799.
Are oilbirds endangered?
The species as a whole is listed as Least Concern, and some colonies contain many thousands of birds. But its ecology makes it fragile in ways a range-wide assessment can hide. The entire breeding population depends on a relatively small number of suitable caves, so disturbance at even one large colony — from tourism, guano mining, or traditional chick harvesting — can affect a big share of a regional population. Oilbirds also need extensive tracts of fruiting forest within nightly flight range of those caves, and deforestation around cave sites can leave a colony unable to feed itself even if the cave is protected. Several important colonies are now inside national parks or reserves.