A diet of fruit is not without potential problems.
Interspecific competition is common, because fruit is
clustered on one or a few trees, which attract crowds
of frugivores. Nutritional balance may be lacking (see
“The Oilbird,” below). Seeds, not surprisingly, tend
to be indigestible, and many fruits tend to be watery,
containing little protein relative to carbohydrate.
Small birds eat small, carbohydrate- rich fruit, and
many must diversify their diets to include protein-
rich animal food. Large frugivores such as toucans eat
many different- size fruits, including those rich in oil
and fat, but they also eat many forms of protein- rich
animal food.
The Oilbird: A Unique Frugivore
The Oilbird (Steatornis caripensis), often called
Guácharo in its native range, from Trinidad and
northern South America to Bolivia, is the only species
in the family Steatornithidae. Its unique evolution was
revealed in a classic two- part study by David Snow. It
is a large nocturnal bird, its body measuring 46 cm
(18 in) in length, its wingspread nearly 1 m (39 in).
The owl- like plumage is soft brown with black barring
and scattered white spots (plate 10- 12). Its broad head
features a large hooked bill and bulging, wide eyes.
Oilbirds are fascinating enough as individuals, but
they come in groups. Colonies are widely scattered
throughout the species’ range, as the birds live in caves,
venturing out only at night to feed on the fat- rich fruits
of palms as well as the fruits of plants belonging to the
laurel family (Lauraceae), often obtained only after
flying long distances from the cave. Fruits are plucked
on the wing: the birds hover at trees, picking off fruits
with their sharply hooked beaks. Oilbirds probably
locate palms by their distinctive silhouettes and are
thought to find aromatic Lauraceae fruits through
olfaction.
Enter an Oilbird cave and be greeted by a cacophony
of sound, a chaotic chorus of growls and screams
emitted by the restless birds (plate 10- 13). In the
dark, dank cave the flapping wings of the disturbed,
protesting hosts conjures up thoughts of tropical
demons awakened. Soon, however, the birds calm and
flutter back to their nesting ledges, snarling as they
resettle. You hear some odd clicking noises punctuating
the din. These vocalizations are one of the features that
make Oilbirds unique. They are among the very few
birds capable of echolocation, the same technique
by which most bats find their way in the dark. Only
a few species of Asian swiftlets, unrelated to Oilbirds,
also echolocate. The clicks are sonar signals, sent out
to bounce off the dark cave walls and direct the birds’
flight. Oilbird echolocation lacks the sophistication
found in that of bats, but it is adequate to keep the birds
from crashing against the cave walls as they fly about
in total darkness.Peccaries and Palm Seeds
Miles Silman and colleagues, in a study at Cocha Cashu Biological
Station in Amazonian Peru, demonstrated how a single species of
seed predator affects the abundance and distribution of a tree species.
White- lipped Peccaries (Tayassu pecari; plate 10- 11) disappeared from
the region in 1978 and reappeared in 1990. Peccaries feed heavily on
large fruits, including those of palms, in which case they act as seed
predators, destroying the seed. Starting in 1978, researchers did transect
counts of the number and spatial distribution of seedlings from the
palm Astrocaryum murumura, a dominant tree species in the region.
The counts were repeated in 1990, when peccaries had been absent for
12 years, and in 1999, after peccaries had reoccupied the region for 10
years. In the years of peccary absence, seedling density of Astrocaryum
murumura increased 1.7- fold. Once peccaries returned, seedling
density declined to what it had been when peccaries had been present
before. The researchers realized that a single species of seed predator,
in this case White- lipped Peccary, exerted a significant influence on the
demography of Astrocaryum murumura. These results emphasize how
species interactions may have major effects on forest species composition.
Peccaries are discussed more in chapter 16.Plate 10- 11. White- lipped Peccary. Photo by
John Kricher.160 chapter 10 tropical intimacy: mutualism and coevolution