Many birds eat plant material, but the hoatzin (Opisthocomus hoazin), a South American bird, is the only avian species known to employ foregut fermentation. Hoatzins are herbivorous; green leaves make up more than 80 percent of their diet

Question

More than a century ago, naturalists observed that hoatzins smell like fresh cow manure, and a study of the crop and lower esophagus of hoatzins revealed that bacteria and protozoa like those found in the rumen of a cow break down plant cell walls, releasing volatile fatty acids that are absorbed in the gut. Considering the abundance of plant material that is available to birds and the widespread use of fermentative digestion by mammals, why do you think the hoatzin is the only foregut fermenter among birds?

Answer 1

Fermentative digestion requires holding material in the gut for prolonged periods of time to allow microorganisms to break down complex molecules. The digesta and the water the gut contains are heavy, and that increased weight is probably incompatible with the demands of flight. Indeed, hoatzins are clumsy fliers and their locomotion is mostly limited to clambering around in trees and bushes. Other birds that feed on vegetation, such as ducks and geese, pass the material through the gut rapidly, making up for low digestive efficiency with high throughput. (If you stroll around an area where a flock of geese has been feeding, you will notice that the ground is covered with goose droppings—convincing evidence that geese have a high rate of throughput!)