Energetics and nutritional ecology of small herbivorous birds

Herbivory is a rare strategy in birds. Only 3% of extant birds exploit plan t material as an energy source. Both plant chemical composition and bird co nstraints associated to flight and digestion may explain why herbivory is s o scarce in small endotherms such as birds. Here we tackle the question of how herbivorous birds meet energy/nutrient requirements when feeding on ext remely poor diets. We focus on Phytotoma rara (Phytotomidae), possibly the smallest herbivorous bird. From our mini-review we conclude that several fe atures such as the fast passage rates, an effective physical breakdown of c ell walls, and constant and high activities of digestive enzymes make avail able the highly nutritious cell contents to the small herbivore, without th e energy costs of anatomical structures for bacterial fermentation or slow passage rates. These are possible crucial adaptations in the evolution of h erbivory in small birds.