Energetics and nutritional ecology of small herbivorous birds

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