Description: Medium-sized duck of approximately 40 cm, highly specialized for mountain river environments. Stiff, long tail used as support in strong currents. Orange-red bill, slender and flexible, adapted for scraping rocky surfaces. Reddish legs. Wing speculum with an iridescent green band visible on the dorsal side of the wing, evident in flight. Wing spurs present. Marked sexual dimorphism. Male with white head and neck with a striking black line pattern, chest and flanks black or variable depending on subspecies, back dark gray finely vermiculated. Female very different, with grayish back and intense rufous to brick-colored underparts. Juveniles duller, grayish with pale barred belly. Subspecies (in Argentina): berlepschi in the northwest, relatively smaller, with white throat and black restricted to the upper chest, more ochraceous underparts. armata in central and Patagonian Andes, male with more contrasted and extensive black and white pattern, darker flanks.

Distribution: Along the Andes mountain range from Venezuela to southern Chile and Argentina. In Argentina it is distributed exclusively in Andean environments, with two well-defined subspecies. M. a. berlepschi inhabits the northwest in Jujuy, Salta, Tucumán, Catamarca, and La Rioja, in rivers of Yungas, prepuna, and high Andean valleys. M. a. armata inhabits the Andes from San Juan and Mendoza southward through Neuquén, Río Negro, Chubut, Santa Cruz, and Tierra del Fuego, associated with mountain rivers in Andean-Patagonian forest and ecotones. Habitat: Fast-flowing mountain rivers and streams, cold and well-oxygenated waters, with rocky bottoms, rapids, waterfalls, and pools. Uses areas with large emergent rocks and steep banks. Mainly present in forested environments such as the Yungas, from foothills to montane forest, and the Andean-Patagonian forest. It may also occur in more open environments such as arid high-altitude valleys above the Yungas, and forest–Patagonian steppe ecotones. In the Yungas it shares habitat with the water ouzel (Cinclus schulzii). Occurs from approximately 300 to 4,000 m depending on the region, reaching highest elevations in the northwest. Resident, with local movements and juvenile dispersal along rivers before establishing territories.

Behavior: Usually in pairs or family groups, strongly territorial along river stretches that it actively defends. Spends much time perched on emergent rocks or feeding. Excellent swimmer and diver, moves against the current using wings and legs. When threatened, usually dives or flies low following the river. Territorial year-round, with frequent interactions between neighboring pairs. Flies low and fast always following the river course. Males produce clear, high-pitched whistles of few notes; females give harsher calls. Also alarm and territorial communication vocalizations.

Diet: Mainly aquatic invertebrates, especially insect larvae attached to rocks. Feeds by diving in fast waters, scraping surfaces with its flexible bill and searching among stones. Diet dominated by mayflies, caddisflies, dipterans, and other benthic macroinvertebrates.

Breeding: Nests during spring and summer, in rock crevices, cavities in banks, or among roots and structures near water; usually lays 3 to 4 eggs; the female incubates and both parents defend the territory and raise the chicks.

Conservation status: Globally categorized as Least Concern, although with a decreasing population trend. In Argentina it is considered Threatened. Although still present across its range, there is evidence of population decline, especially in the Patagonian subspecies. Main threats include river alteration by human activities, tourism in waterways, dams, introduction of exotic fish such as salmonids that compete for food, and predation by invasive species such as the American mink. Conservation of well-preserved mountain rivers is key for its survival, and there are protected populations in several natural areas of the country.


Author of this compilation: Diego Carus y María Belén Dri – 03/04/2026