The Oilbird is a large, strange, vegetarian nightbird only remotely related
to other families in the Order Caprimulgiformes. It roosts and breeds in
selected caves that are used by the species for eons. Brad Schram
took this nice shot of several roosting Oilbirds (left or above)
at a traditional site on Trinidad. The Oilbird ranges locally across much
northern South America and down the Andes all the way to Bolivia. It is
a bird of tropical and subtropical primary forests. Colonies leave the
caves at night to fly to fruiting trees. Figs are often consumed but the
Trinidad population has been documented eating over 36 kinds of fruit.
The seeds of these fruit are spread widely after passing through the bird's
system, and the Oilbird is thus an important component of the tropical
ecosystems. The forest depends on these fruit eaters -- and other animals
with similar behavior such as fruit bats -- to continue in existence.
Biochemical evidence reviewed by Sibley & Ahlquist (1990) and Sibley & Monroe (1990) led them to place the Oilbird at the beginning of their arrangement of nightjars and allies, just before the potoos. They appear rather distantly related at best. Indeed, the Oilbird evolved long ago. There are fossils going back nearly 50 million years from the Green River Formation in Wyoming. They may have once been widely distributed in the early radiation of birds across the Northern Hemisphere (Thomas 1999). Oilbirds echo-locate (like bats) inside caves but use their huge eyes once they leave the communal roosts. They apparently have excellent night vision. Rows of bristles protect the eyes both above and below. They fly with fast with primary tips widely spaced (like New World vultures) but have the ability to fly slowly or even hover. They can range quite a distance in search of fruit. Radio telemetry studies have shown their normal range per night is 40 kms (25 mi) but they have traveled as far as 150 km (93 mi) in a single night. They may find fruit by smell. Studies have shown that all the fruit they eat is spicy or aromatic when ripe. The Oilbird colonies in Trinidad are permanent and sedentary; the only movement is post-breeding dispersal. It appears the individual birds may use the same roosting spots year-after-year. Compare Brad Schram's photo of a roosting spot in Dunston Cave, Trinidad, with my own photo (below) of a roost spot in that same cave, taken almost 7 years later. Flash photography is not permitted here, so these are hand-held shots in the beam of a flashlight. |
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In the Andes some caves may be used only in the breeding season. Everywhere they have been studied nesting is initiated at the end of the dry season. This can be as early as March (or as late as July in Trinidad). I visited an Oilbird cave in July 1975 in the western Andes of Colombia but no birds were present that day. Pairs are monogamous on a long-term basis. The female lays 1-3 eggs and these hatch in about a month. It may take another six weeks to fledge the youngster which has no distinct juvenal plumage. Very young birds are very fat and full of oil; the original name comes from these babies. The feathers of an Oilbird are unique and unlike any other bird, but
few folks would know this. I have heard a story -- perhaps apocryphal --
about an Oilbird feather and a birding trip to Attu Island at the end of
the Aleutian chain in Alaska. Indeed, it is so far out there that the date
line has a huge kink just to keep Attu in the same day as the rest of the
United States. For a couple decades expeditions were mounted to this remote
barren island in spring in hopes of finding Siberian vagrants. The hope
was to find first North American records among these migrants. Attours
(the organizing company) always had several "big name" leaders along. In
any event, one year someone brought an Oilbird feather surreptitiously
and arranged for one of the less experienced birders on the island to "find"
it in the field and bring it in to dinner. When she did so, the feather
was passed around from big name to big name and nobody had any clue what
it was -- except that it "must" be a first North American record, whatever
it was. Or at least that's how it went until the feather reached Laurence
C. Binford, professional ornithologist and at that time the director of
the California Academy of Sciences. Laurie said calmly "oh, its an Oilbird"
and there went the whole practical joke. Damn ornithologists....
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| Photos: The colony of Oilbird
Steatornis caripensis (top photo) was photographed by Brad Schram on
29 Feb 2000 at Dunston Cave, Asa Wright Nature Center, Trinidad. My own
photo of part of the colony (middle shot) is at that very same case on
25 Dec 2006.
All photos © D. Roberson 2006, except
the shot credited to Brad Schram, who holds that copyright; used with permission;
all rights reserved.
Bibliographic note: Family Book: Literature cited: Cleere, N. 1998. Nightjars: A Guide to the Nightjars, Nighthawks, and their Relatives. Pica Press, London [co-published in U.S. by Yale Univ. Press, New Haven CT] Illustrated by Dave Nurney.TOP |
