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Version 1.0

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Brazilian Merganser Mergus octosetaceus

Ivana R. Lamas and Líiva V. Lins
Version: 1.0 — Published April 28, 2009



The species spends the days interspersing foraging and resting periods. Generally, they rest on rocks or branches which are projected out of water, as well as on river beaches. They also can stay in the water motionless or with very slow movements.

They remain always alert. In any sign of danger, they immediately go into the water. Being in the water, depending on the risk, they either swim quickly down or up stream, or take off flying away. Most of the times, they fly close to water, following the water course; nevertheless they can also fly higher (about 20 m high). For long distance movements the flights can be even higher, always following the water course. The adults never fly when they have flightless young. In this case, the family attempt to escape by taking cover in vegetation along the shore vegetation, or swimming quickly away. The parents maintain the young between them when swimming and they constantly keep guard over the offspring.

Brazilian Merganser is a proficient swimmer and even the young easily pass through strong rapids. There is indication the total moult occurs soon after the reproductive season, as it happens with other mergansers (Silveira and Bartmann 2001), but this requires confirmation.


We believe Brazilian Merganser do not migrate seasonally between different regions, basins or sub-basins. In the rainy season, when the water turns into muddy, it shows a preference for upper streams where the small tributaries are clear. In this period, the birds are more difficult to observe.

The couples keep their territories well defended all over the year. Although territorial disputes have been recorded, fights between pairs established in adjacent territories are rare (Silveira and Bartmann 2001). Recently, we have observed some Brazilian Merganser pairs in territories we believed already to be occupied by a different pair. Hopefully the results from the project of marking and monitoring, still in developing, will clarify these issues on territory overlap.

Sexual Behavior

Brazilian Merganser is monogamous. The same pair seem to live together for a long time, if not throughout their lives. However, since their individual identification is very difficult, only with the advance of the marking-monitoring project (Lins et al., in press), will we be able to confirm whether the couples we observed year after year are really composed by the same individuals.

Males court females on the water; the copulation is also on the water. Pair of Brazilian Mergansers "talking". Photo: Carlos E.A. Carvalho/Instituto Terra BrasilisWhen the female is ready to accept the male, she lowers her head and extends the body horizontally. Then the male mounts the female grasping her crest with the beak. The copulation can last for 15-25 seconds, notably longer than in other mergansers (Silveira and Bartmann 2001).

Social and interspecific behavior

There is very little information on the interspecific interactions and their consequences for the Brazilian Merganser populations.

Some other bird species commonly observed in the vicinity of the Brazilian Merganser are kingfishers (Alcedinidae), Brazilian Teal (Amazonetta brasiliensis), Pied-billed Grebe (Podilymbus podiceps), Anhinga (Anhinga anhinga), and Muscovy Duck (Cairina moschata). Nevertheless, there is no evidence of direct interaction between these species and the merganser.

The Neotropical River Otter (Lontra longicaudis) also is commonly seen in rivers where the Brazilian Merganser occus. It may compete with Brazilian Merganser for the same food resources since both diets are based on fishes. It is also important to evaluate the probability of predation on Brazilian Merganser by the otter.


Little information. It has been speculated that other mammals and birds recorded in the Serra da Canastra region, based on their diet, size and habitat, might prey on Brazilian Merganser (Lamas and Santos 2004). Among mammals, Rogério de Paula (pers. comm., 2002) cites Puma (Puma concolor), Ocelot (Leopardus pardalis), Margay (Leopardus wiedii), Jaguarundi (Herpailurus yagouaroundi), Maned Wolf (Chrysocyon brachyurus), Crab-eating Fox (Cerdocyon thous), Tayra (Eira barbara) and Neotropical River Otter (Lontra longicaudis) as potential predators on Brazilian Merganser. Among raptors, Dante Buzzetti (pers. comm., 2002) highlights Black-chested Buzzard-Eagle (Geranoaetus melanoleucus), Crowned Eagle (Harpyhaliaetus coronatus), Aplomado Falcon (Falco femoralis), Grey-headed Kite (Leptodon cayanensis), Black-and-white Hawk-Eagle (Spizastur melanoleucus), Collared Forest-falcon (Micrastur semitorquatus) and Great Horned Owl (Bubo virginianus) as potential predators.

L. V. Lins and colleagues (pers. obs., 2007) have observed a Great Black Hawk (Buteogallus urubitinga) stooping over a merganser in Serra da Canastra. The merganser evaded capture by diving under the water each time the hawk got close.

Partridge (1956) also drew attention to the Black-and-white Hawk-Eagle as amongst the most dangerous predators of Mergus octosetaceus in Argentina. The same author highligthed the “Dourado” (Salminus maxillosus), one of the most voracious fish of the upper Paraná, as a potential enemy to young ducklings of any species. We saw dogs turning excited after hearing Brazilian Merganser play-back vocalizations. This may be evidence of a possible agonistic interaction between the two species.

Recommended Citation

Lamas, I. R. and L. V. Lins (2009). Brazilian Merganser (Mergus octosetaceus), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/nb.bramer1.01