The New Neotropical Companion

Oilbirds are closely related to the diverse family of

nightjars (Caprimulgidae; chapter 15), of which the

Eastern Whip- poor- will (Caprimulgus vociferus) is

a common example. However, nightjars, unlike the

Oilbird, are all insectivorous, are not colonial, and do

not live in caves. How did the Oilbird evolve frugivory,

sociality, and its cave- dwelling habit?

David Snow offered an evolutionary scenario for

Oilbird evolution that begins with a critical diet shift

from insects to fruit. Snow hypothesizes that Oilbirds

were originally “normal” nightjars feeding on insects.

However, large fruits offer a potentially exploitable

high- calorie resource, especially to a nocturnal bird.

Furthermore, there are few large bats in the Neotropics.

That is not the case in the Paleotropics, where many

species of megachiropteran bats (flying foxes) feed on

large fruits. In the Neotropics, many microchiropteran

bats are frugivores, but none are comparable in size to

an Oilbird, so they cannot eat the fruits that Oilbirds

devour. Oilbirds thus became Oilbirds when their

ancestors shifted to a diet of fruit (fig. 10- 1).

Frugivory initiated a cascade of adaptations that

profoundly affected the evolution of the Oilbird (plate

10- 14). The birds’ olfactory sense became enhanced

as it provided an advantage in locating aromatic

fruits. Social behavior began to evolve because fruits

are a patchy resource and birds would tend to come

Plate 10- 12. Oilbird on its nest. Photo by Edison Buenaño.

Plate 10- 13. Dunston Cave in Trinidad, where Oilbirds are

easily seen. Note the spindly plants that have germinated at

the lower left. They sprouted from seeds dropped by the birds.

Photo by John Kricher.

Oilbird Evolution According to (mostly) David Snow

Vision - palms

Olfaction - laurels

Longer development

period before fledging

More highly conspicuous

ground nests due to

seed defecation

Sociality

Nest colonially in caves

Echolocation

Larger clutch

sizes

Predator free

100 days from egg

Ti

me?

Fledging

Insect diet Fruit diet Selects against

territoriality

Occupy cave and same nest all year (mating occurs for life?)

Figure 10– 1. This diagram shows the hypothesized evolution of

Oilbirds. Reprinted with permission from Kricher, John. Tropical

Ecology. Princeton, NJ: Princeton University Press, 2011.

Plate 10- 14. Oilbirds have hooked beaks, which they use to

pluck fruits as they hover. Photo by John Kricher.

chapter 10 tropical intimacy: mutualism and coevolution 161