The Brazilian merganser (Mergus octosetaceus) is a critically endangered South American diving duck, with fewer than 250 individuals left in the wild. It is characterised by its lustrous dark-green hood, a spiky crest, and a long, thin, black bill with tooth-like edges for catching fish in clear, fast-flowing rivers. A major threat is habitat degradation and loss due to agriculture, logging, cattle ranching, and the construction of dams. The Brazilian merganser is monogamous, forms strong pair bonds, and nests in tree cavities or rock crevices.
