together at fruiting trees. More important, however, is
the fact that a diet of palm and laurel fruit, though rich
in calories, is nutritionally unbalanced. Consequently,
nesting time is prolonged, and incubation of each egg
lasts just over one month. Once Oilbirds hatch, they
fatten up immensely in the nest due to a buildup of
fat from the oily fruits. But they take a long time to
acquire sufficient and proper protein to grow bones,
nerve, and muscle. Two months after hatching, a
juvenile may weigh 1.5 times either parent’s weight,
but it still has not left the nest. The name Oilbird refers
to the fact that juveniles put on so much fat that they
can be boiled down to render the oil. Indigenous
people occasionally used them for torches since they
burn so well! The total time it takes an Oilbird to go
from newly hatched egg to fledging and independence
is nearly 100 days, compared with about 30 days for
insectivorous nightjars. It takes six months for a clutch
of four Oilbird eggs to develop from embryo to flight.
Such an extended development time makes it risky
(even absurd) to nest on the forest floor, the traditional
nightjar nesting site. Snow notes that the defecated
seeds that would surround a ground nest would serve
to bring attention to nesting birds. Cave dwelling offers
more protection for the nest, but caves are also very
patchily distributed resources. Once Oilbirds took up
cave dwelling, they became colonial, social species.
Cave dwelling selected for the development of
echolocation and also for a larger clutch size, one more
typical of birds in the temperate zone. Most tropical
birds lay only one or two eggs in a given nest, but birds
in the temperate zone have clutches often of four or five
eggs or more. Predator pressure is likely to be a major
reason for the small clutch sizes in the tropics, since
nests can be more secretive if there are fewer mouths
to feed. Given the safety of caves, Oilbird nests are not
under severe predator risk, and clutch size is normally
four eggs. The nest is built up with droppings from
the birds and located on a cave ledge. Thin, yellowish,
light- starved seedlings sprout from defecated seeds
around the nest. Oilbirds are thought to pair for life.
The Oilbird is an important seed- dispersing species.
In an exhaustive study centered in Cueva del Guácharo
(Guácharo Cave) near the town of Caripe, in Monagas,
a mountainous, heavily forested region in northeastern
Venezuela, Ricardo Roca, using radio- tagged Oilbirds,
demonstrated that the birds have home ranges that
encompass up to 96.3 km^2 (37.2 mi^2 ) and may have
to fly up to 150 km (nearly 95 mi) between feeding
sites. Indeed, dispersing individuals fly even farther in
search of food, up to 240 km (150 mi) in a single night.
Given that an adult Oilbird requires approximately
50 fruits daily, Roca calculated that the entire colony
he studied collectively regurgitated approximately
15 million seeds each month, a biomass of about 21
tons of seeds. Roca estimated that about 60% of the
seeds were dispersed in forest. Oilbirds are important
species in maintaining the plant biodiversity of the
forests in which they forage, and as such, merit strict
conservation measures, especially around their caves.Animal Diversity at Fruiting Trees
Fruit is an important resource in both mountain
and lowland forests. Nathaniel Wheelwright and his
colleagues did a comprehensive survey of fruiting trees
and fruit- eating birds at Monteverde Cloud Forest in
Costa Rica and learned that 171 plant species bore
fruit that was fed upon by 70 bird species. Some birds
depended heavily on fruit, others consumed it more
casually. Among the birds were three woodpecker
species, nine tyrant flycatchers, eight thrushes, eight
tanagers, and nine finches, as well as toucans, pigeons,
cotingas, and manakins. Though some birds were
observed to feed on fruit only rarely, it was clear that
fruit represents an important resource for a large
component of the avian community. In comparison,Plate 10- 15. Masked Tityra male at its nest cavity. This species
has been shown to be an efficient seed disperser. Photo by
James Adams.162 chapter 10 tropical intimacy: mutualism and coevolution