Most dispersal mutualisms between animals and plants depend on the inclusion of plant propagules (seeds, pollen) as a part of the animal food, and result from the animal feeding on the plant (Janzen 1985). A major constraint on the evolution of a tight mutuatistic relationship between animals that gather food and plants that provide this food is that no single plant part produces an adequately balanced set of nutrients for the consumer and, as a consequence, the animal must build up a diverse diet in terms of both plant species and plant parts. The consequences of these nutritional constraints for the animals involved have been repeatedly documented (e.g. Belowski 1978, Westoby 1978, Bergeron & Jodoin 1984, Karasov 1985), and involve, as a generalization, the need to build up a diverse diet including 'minor' components that might be difficult to obtain. This has consequences for the patterns of habitat use, time budget, digestive physiology or body condition.

Frugivorous birds feeding on fleshy fruits experience nutritional constraints quite similar to those of herbivores feeding on vegetative plant parts (Foster 1978, Moermond & Denslow 1985). The fruit pulp is extremely scarce in protein and, if the amount present is adequate, it is usually imbalanced with other fractions, such as lipids or carbohydrates, resulting an inadequate food even when ingestion rate is not limiting (Foster 1978, Robbins 1983). The need to obtain minor nutrients (minerals, vitamins) and the presence of secondary plant metabolites, acting as digestion inhibitors or poisons, might impose deviations from the predictions derived from energetic models by creating partial preferences (Westoby 1978). Shortage of specific nutrients may result in individual preferences for particular meals, i.e., combinations of food items that better supply the nutritional needs (Rozin & Kalat 1971, Clark 1980). In addition, the seed(s) acts as an indigestible ballast decreasing the profitability of the nutrients within the fruit (Herrera 1981, Levey 1986). Therefore, a short retention time of the ingesta is the most commonly reported characteristic of frugivores. It can be seen as an adaptation to deal with digestive bottlenecks, arising when the caloric demand is fulfilled but the nitrogen or mineral need is not supplemented because food processing is limiting (Sibly 1981).