A bird that eats like a cow
Deep in the swamps and riverine forests of the Amazon and Orinoco Basins lives the Hoatzin (Opisthocomus hoazin), a bird with a digestive system unlike any other. It is the world's only known folivorous (leaf-eating) bird that uses foregut fermentation, a strategy for breaking down tough plant matter that is common in ruminant mammals like cattle but unique among avian species. A study in Venezuela found its diet consists of 82% leaves, 10% flowers, and 8% fruit. This specialized diet requires a specialized gut. The Hoatzin's digestive tract is dominated by an enormous, muscular crop and lower esophagus, which together account for 77% of its total gut capacity. This fermentation chamber is so large it displaces the bird's flight muscles and keel, making it a clumsy, weak flier.
The fermentation process relies on a complex community of bacteria that break down cellulose in the leaves. Analysis of the crop's microbial population has identified nine different phyla of bacteria, with Firmicutes (67%) and Bacteroidetes (30%) being the most dominant. Many of these bacterial lineages are novel to science. This microbial action, combined with muscular grinding against the crop's hard, cornified interior lining, effectively digests up to 70% of the plant fiber. The process is slow, with a single meal taking up to 45 hours to pass through. As a byproduct, the fermentation releases methane and other volatile aromatic compounds, which the bird expels by burping, earning it the local nickname "stinkbird" due to its manure-like odor.