Very few bird species subsist mainly on leaves as a food source. Those that do usually pay a significant price because of the high fiber content and diluted energy availability of leafy foods. The trade-offs may involve larger size, flightless-ness, and low activity to conserve energy. There may also be elaborate modifications of the digestive tract. The New Zealand takahe (Porphyrio mantelli), whose dietary mainstay is alpine tussock grasses, is flightless. It has an inefficient digestive system and must eat almost continuously during its waking hours to ensure adequate nutrition. The leaf-eating hoatzins (Opisthocomus hoazin) of tropical South America lodge passively in dense riverside thickets for safety and are poor fliers. Their crops are enlarged, having become extra stomachs full of symbiotic bacteria able to digest the tough, fibrous cell walls of leaf tissues.
The plantcutters, though, seem not to have paid a high price for their choice of diet. The secret lies in the species'
feeding methods and digestive system. Chilean biologists recently undertook several studies of food intake and processing in the rufous-tailed plantcutter. Data support the efficiency of the bird's feeding and digestion, and this likely applies to the other plantcutter species. Plantcutters chew their leafy food into a pulp to rupture the tough plant cell walls and free the nutritious cell interiors for digestion. Food passes rapidly through the digestive system, which has little in the way of elaboration (although the intestine is abundantly supplied with mucous cells along its length, concentrating them toward its nether end). This combination of chewing, rapid passage of food through the digestive system, and efficient digestion allow the bird to process hefty amounts of plant material over shorter times and thereby maintain a high metabolic level.
Plantcutters spice up their diets with some intake of fruits and insects, but leaves of many plant species, depending on availability and type of habitat, are the mainstay. Rufous-tailed plantcutters have developed an affinity for cereal leaves, among them wheat and oat.
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