DIGESTIVE EFFICIENCY AND RATE OF FOOD PASSAGE IN OILBIRD NESTLINGS

Oilbirds (Steatornis caripensis) of South America are unusual in raising their young exclusively on a fruit pulp diet. In order to determine the ability of Oilbird nestlings to extract energy and nutrients from the diet, we studied their digestive function under laboratory conditions. We used total balance feeding trials to determine the Metabolizable Energy Coefficient (MEC*) and digestibility of the components of an avocado-based diet. Oilbird nestlings extract an average of 80% of dietary lipids and 88% of dietary nitrogen. They also metabolize some of the ingested fiber. Mean MEC* (64%) was similar to that of other frugivores. Nitrogen requirements for maintenance (determined from the regression between nitrogen intake and excretion) were considerably lower (less than 0.075 g N kg-0.75 day-1) than estimates for non-passerines. Efficient utilization of dietary nitrogen is considered an essential adaptation for nestlings on a nitrogen-poor diet. In contrast to all other frugivores so far studied, Oilbird fledglings have a long digesta retention time. Mean retention time of plastic markers mixed in the experimental diets averaged 377 min. This may be attributable to the high lipid content of their diet. Our results help to explain the nutritional and energetic adaptations to frugivory in this species.