Lake Duck

Identification

The lake duck is a small, compact waterfowl measuring 36–46 centimetres in length, with females weighing 510–700 grams and males slightly heavier at 600–850 grams. Males can be distinguished from the similar Andean duck by their smaller size and noticeably flatter head profile. This stiff-tailed duck, also known as the Argentine blue-bill, is endemic to South America and represents one of the continent's most unusual avian species.

Beyond its modest external appearance, the lake duck is famous in the animal kingdom for an extraordinary biological distinction: males possess the largest avian penis relative to body size on record. This reproductive organ, typically coiled when flaccid, can extend to nearly the bird's full body length when erect—a remarkable adaptation theorized to result from intense sexual competition among these highly promiscuous birds, functioning somewhat like a bottle brush to remove sperm from previous matings.

Range & Habitat

The lake duck is naturally distributed across much of southern South America, occurring in Argentina, Brazil, Chile, Paraguay, and Uruguay. The species has also been introduced to the Falkland Islands. As a partially migratory species, it shows varying breeding patterns depending on location, with the Argentine population breeding between October and January.

This duck favours freshwater environments rich in aquatic vegetation, particularly wetlands and lakes where dense plant growth provides both food and cover. The species maintains a stable population estimated between 6,700 and 67,000 individuals, with no major conservation threats identified. It is listed as a species of least concern on the IUCN Red List as of 2016.

Behaviour & Diet

The lake duck's diet remains poorly documented but is believed to comprise small invertebrates, seeds, and plant remains, likely gleaned from the vegetated wetland habitats it frequents. Males are notably vocal during breeding season, producing distinctive popping noises and "mechanical rustling noises" as part of their courtship display.

The species exhibits a complex mating system involving forced copulation attempts by males. However, females possess a remarkable counter-adaptation: their reproductive tract spirals in the opposite direction to the male's corkscrew penis, allowing them to retain control over fertilization. This sophisticated reproductive geometry means that most forced copulation attempts do not result in successful breeding, effectively giving females the final say in reproduction despite male aggression.

Best Time & Places to See It

The breeding season from October to January in Argentina offers the best opportunity to observe males in display, when their vocal activity peaks. Lake ducks are very widespread across their South American range, making them accessible to travellers visiting wetland systems throughout Argentina, Brazil, Chile, Paraguay, and Uruguay.

Look for this species in vegetated freshwater lakes and extensive wetland complexes where emergent and aquatic plants are abundant. The stable population and lack of major threats mean sightings are relatively consistent throughout the year in suitable habitat.

Photography Tips

The lake duck's small size and preference for vegetated habitats can make photography challenging. Position yourself quietly near lake margins where vegetation is less dense, allowing clearer sightlines to feeding birds. Early morning and late afternoon often provide the best lighting and greatest activity levels.

During breeding season, patient observers may be rewarded with shots of males displaying with their distinctive popping and mechanical sounds—look for animated posturing and head movements that accompany these vocalizations. A telephoto lens will be essential to capture detail while maintaining distance from these wary waterfowl.