Zoologger: formidable Guácaro

Zoologger: The troglodyte bat bird of South America

• 12:00 10 February 2010 by Michael Marshall
• For similar stories, visit the Zoologger Topic Guide

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world.

Species name: Steatornis caripensis

Habitat: Caves, in woodland areas in the north of South America

"Raucous shrieking and frightful retching… which might express the sufferings of sea-sick demons." Not a passage from Milton, though this description by the early 20th-century zoologist John Golding Myers does describe his entry to a kind of earthly hell: a cave of roosting oilbirds.

This split-personality, cave-dwelling oddity, known to North Americans as the guacharo, doesn't seem to know whether it is bat or bird. It echolocates like a bat to perceive its surroundings, but as well as this crude form of sonar, the oilbird has the most sensitive eyes of any vertebrate.

It has feathers and a wingspan of 90 centimetres. It sports a menacing hooked beak. OK – it's a bird, though a weird one. And new evidence suggests that it plays a major role in preserving the forests where it lives.

As Myers noted, oilbirds spend much of their time squabbling in caves, in colonies numbering up to 20,000 birds. Because of the immense numbers living there, the floor is carpeted with guano, which supports a host of insects and other small animals. The birds also put the guano to good use during the breeding season: they build nests with it.

Flying oil wells

Oilbirds get their name from their diet. Most of their relatives in the order of Caprimulgiformes or nightbirds hunt insects, but the oilbirds have shunned bug-munching and gone vegetarian. They eat only fruit, and a particular favourite is the fruit of the oil palm, a plant that is harvested by humans for the fatty oils it produces. The younger birds become so tubby from this diet that people used to catch them and render them down as a source of fat – hence the name. Oilbirds feed at night, sallying out into the forest and crossing tens of kilometres to gorge themselves – just like many bat species.

Oilbirds' eyes are some of the most sensitive known. Within the retina, the rods – the light receptors used for night vision – are jammed together, with around 1 million of them to every square millimetre.

However, there is a trade-off for their night-sight powers. Because so much of the eye is devoted to the ultra-sensitive rods, there is little space left for the cones, the other kind of light receptor, which deal with fine details and colour. So it seems likely that oilbirds have fuzzy vision and can see little or no colour. This is unusual for an animal that lives on fruit: most such beasts have excellent colour vision to help them spot ripe specimens.

Cave shift

It used to be thought that oilbirds spent all the daylight hours in their home caves, and consequently never saw the sun. But a new study has revealed that to be a myth.

Richard Holland of the Max Planck Institute for Ornithology in Radolfzell, Germany, and colleagues travelled to an oilbird cave in Venezuela, where they tagged 12 of the birds with miniature GPS trackers.

It turned out that the birds only spent about one day in three in the caves, spending the other two sitting quietly in trees. This might dispel some of their mystique, but it highlights their ecological importance.

If oilbirds really had spent all their days in the caves, any seeds they ate would have ended up on the cave floor and would never grow into adult plants. But in fact, many of them end up on the forest floor, far away from their parent plant, where they have a much better chance of surviving.

Holland and his colleagues noted that the birds are very active while in the caves, possibly because they are constantly defending their roosting spots and maintaining the pecking order. By contrast, when they are sitting in the trees they are virtually stationary.

It might be risky to roost out in the open, but it looks like the oilbirds leave their caves to get some peace and quiet.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0008264