Diet and Foraging

Oilbirds are strictly frugivorous. Their diet is mainly composed of fruits of only three plant families: Burseraceae, Lauraceae and Palmae. This diet seems to be narrow across their whole distribution range (Bosque et al. 1995). Oilbirds rely only on fruit for maintenance as well as for raising their nestlings (Bosque et al. 1995). Throughout their growth period they deposit fat reserves. Nestlings become approximately 50% heavier than adults before fledging (Snow 1961, White 1974, Tannenbaum and Wredge 1978, Ramírez 1987).

Oilbirds track changes in fruit availability (Bosque et al. 1995). In Venezuela, the period of greatest Lauraceae abundance occurs in the months April-June when most young are in the nest (Snow 1962); thus, they feed on palms more consistently during the nonbreeding season (Bosque et al. 1995). However in Trinidad, the environment is less seasonal compared to Venezuela and Oilbirds tend to have a longer breeding season; young birds are in the nest in any month of the year but there is a marked peak of laying in the months December-May (Snow 1961, 1962).

Bosque et al. (1995) showed that Oilbirds use fruits which have a high energy or high nitrogen content or those which have both, but does not include in its diet species that have neither. Oilbirds eat fruits with high lipid content, low water content and high energy content unit per unit of dry fresh mass. Frugivores that process succulent, energy dilute pulps must consume daily quantities of fruit approaching double their body mass for maintenance alone (Moermond and Denslow 1985, Studier et al. 1988). Adaptations to an energy rich diet is one of the key features that allows Oibirds to subsist on a totally frugivorous diet.