Introduction
The Scaly-sided Merganser (Mergus squamatus) is a specialised fish-eating diving duck of East Asia. It breeds along clear, fast-flowing rivers in the Russian Far East, north-eastern China and the northern Korean Peninsula, and migrates mainly to river systems in central and southern China. The species is listed as Endangered on the IUCN Red List and depends on healthy rivers, mature riparian woodland and relatively undisturbed breeding and wintering sites.
Physical Description
The Scaly-sided Merganser is a long-bodied duck with a narrow, serrated bill adapted for catching aquatic prey. The adult male has a glossy dark green-black head and neck, a mostly white body, dark wings and distinctive black-edged scaling across the flanks and lower body. A wispy crest extends backwards from the head, particularly in breeding males.
Females and juveniles are less strongly patterned. They have warm brown heads, grey-brown upperparts, paler underparts and a noticeable pale throat or chin area. The scaly flank pattern is generally less pronounced than in adult males. Both sexes have a long, slender bill and reddish-orange legs.
The species is sexually dimorphic, with adult males being more contrasting and ornate than females. Young birds resemble females but usually appear duller and less distinctly scaled until they acquire more adult plumage.
Range and Habitat
The breeding range is concentrated in the Sikhote-Alin and adjacent regions of the Russian Far East, the Changbai Mountains and other parts of north-eastern China, and northern areas of the Korean Peninsula. The species is a migrant, although some individuals may remain within or near the breeding range during winter.
During the non-breeding season, most birds use rivers in central and southern China. Smaller numbers occur in Korea and Japan, and the species may also be recorded elsewhere in eastern Asia. Wintering birds are widely dispersed rather than concentrated at a single major wetland.
Breeding habitat consists mainly of clear, cool, fast-flowing rivers bordered by mature temperate or submontane forest. Suitable stretches often contain riffles, shallow runs, rocky sections, islands or gravel bars. The birds nest in natural tree cavities, so the availability of large trees near suitable rivers is important.
Wintering birds continue to favour relatively clean, flowing water, although they may use wider and slower rivers than those occupied during the breeding season. Low human disturbance, exposed gravel bars, open water and access to suitable feeding areas are important habitat features.
Behaviour and Diet
The Scaly-sided Merganser is generally wary and easily disturbed. It spends much of the day on or near the water and dives to pursue prey beneath the surface. Its serrated bill helps it grip slippery prey in swift water.
The diet includes small fish and aquatic invertebrates. Invertebrate prey may include aquatic insect larvae and other riverine animals, with the balance varying according to local conditions, water depth and season. Birds can adjust their foraging tactics between shallow and deeper water.
Breeding birds use tree cavities for nesting. The loss of mature trees can therefore reduce nesting opportunities even where the river itself remains apparently suitable. Artificial nest boxes have been used as a conservation measure in areas where natural cavities are scarce.
The species migrates between its northern breeding rivers and mainly Chinese wintering rivers. Tracking and survey work has shown that some individuals return to the same wintering catchments, but the full migration network remains incompletely documented. During winter, birds may occur singly or in small groups, although larger groups are also recorded at suitable sites.
Identification Tips
- Look for a long, slim diving duck with a narrow reddish bill and a backward-projecting, wispy crest.
- Adult males show a glossy dark head and neck, white body, dark wings and conspicuous black-edged scaling on the flanks.
- Females and juveniles have a warm brown head, pale throat and grey-brown body. The scaled flanks can be difficult to see on a moving bird.
- The species is most likely to be encountered on clear rivers with riffles, gravel bars, rocks or quiet vegetated margins, rather than on heavily disturbed open water.
- The closely related Common Merganser, also called Goosander, is a useful comparison. Female Scaly-sided Mergansers usually show more obvious flank scaling, while female Common Mergansers generally have a cleaner division between the brown head and pale body.
- Red-breasted Mergansers can also resemble them, but usually have a more ragged crest and, in adult males, a darker and more strongly patterned reddish-brown breast. Local range and river habitat are also useful clues.
Conservation Status
The Scaly-sided Merganser is classified as Endangered on the IUCN Red List. Published population estimates vary because the species is difficult to survey across remote breeding rivers and widely scattered wintering sites. It is considered to have a small global population, with historical declines linked to major changes in riverine forest and waterways.
Important threats include:
- Deforestation and the removal of large trees that provide nesting cavities.
- Dam construction and other river engineering that replace flowing water with deep, still reaches.
- Sand and gravel extraction, which can remove feeding and roosting bars.
- Pollution from industrial, domestic and agricultural sources.
- Disturbance along breeding and wintering rivers.
- Illegal hunting and shooting.
- Entanglement or drowning in fishing nets.
Conservation measures include protecting breeding rivers and mature riparian forest, maintaining environmental flows, safeguarding gravel bars and other key wintering features, reducing pollution and disturbance, and improving enforcement against illegal hunting and harmful fishing practices. Artificial nest boxes, population surveys, satellite or other tracking methods, and coordinated monitoring by researchers and local birdwatchers can help fill important knowledge gaps. Because the species crosses national borders, effective conservation requires cooperation among countries along its breeding and migration routes.
Frequently Asked Questions
Where does the Scaly-sided Merganser live?
It breeds in the Russian Far East, north-eastern China and the northern Korean Peninsula. Most birds migrate to rivers in central and southern China for winter, with smaller numbers wintering in Korea and Japan.
What does the Scaly-sided Merganser eat?
It dives underwater to catch small fish and aquatic invertebrates, including river-dwelling insect larvae and other small aquatic animals.
Why is it called the Scaly-sided Merganser?
The name refers to the adult male’s distinctive flank pattern, in which dark markings create a scaled appearance against the pale body.
Where does the species nest?
It nests mainly in natural cavities in mature trees beside suitable rivers. Artificial nest boxes may provide additional nesting opportunities where old trees and natural cavities are scarce.
Is the Scaly-sided Merganser endangered?
Yes. The species is currently classified as Endangered on the IUCN Red List. Its main threats include river modification, loss of riparian forest, pollution, hunting and entanglement in fishing nets.
How can birdwatchers help with its conservation?
Responsible birdwatchers can submit well-documented sightings to recognised recording schemes, avoid disturbing birds on sensitive rivers and follow local access and wildlife regulations. Records from areas that are poorly surveyed can help improve knowledge of distribution and seasonal movements.
References
- BirdLife International. 2017. Mergus squamatus. The IUCN Red List of Threatened Species. Available through the IUCN Red List species database. Accessed 8 September 2026.
- Gill, F., Donsker, D. and Rasmussen, P. (eds.). 2026. IOC World Bird List, version 15.2. International Ornithologists’ Union. Available at: https://www.worldbirdnames.org/ Accessed 8 September 2026.
- Solovyeva, D. V. 2013. Foraging behaviour and daily time budget of Scaly-sided Merganser Mergus squamatus breeding on Iman River, Russia. Wildfowl 53: 205–214. Available at: https://tidsskrift.dk/Wildfowl/article/view/153986
- Solovyeva, D. V., Shokhrin, V. P., Vartanyan, S. L., Dondua, A. S. and Vartanyan, A. S. 2014. The population size and breeding range of the Scaly-sided Merganser Mergus squamatus. Bird Conservation International 24(4): 393–405. https://doi.org/10.1017/S0959270913000610
- Barter, M., Zhuang, X., Wang, X., Cao, L., Lei, J., Solovyeva, D. and Fox, A. D. 2014. Abundance and distribution of wintering Scaly-sided Mergansers Mergus squamatus in China: where are the missing birds? Bird Conservation International 24(4): 406–415. https://doi.org/10.1017/S0959270913000622
- Zeng, Q., Lei, G., Zeng, B. and others. 2015. Using Species Distribution Model to Estimate the Wintering Population Size of the Endangered Scaly-Sided Merganser in China. PLOS ONE 10(1): e0117046. https://doi.org/10.1371/journal.pone.0117046
- Lei, J., Zhuang, X., Barter, M. and others. 2015. Gravel Bars Can Be Critical for Biodiversity Conservation: A Case Study on Scaly-Sided Merganser in South China. PLOS ONE 10(6): e0127387. https://doi.org/10.1371/journal.pone.0127387
- EAAFP. 2016. Scaly-sided Merganser Single Species Action Plan 2016–2025. East Asian-Australasian Flyway Partnership. Available at: https://www.eaaflyway.net/wp-content/uploads/2017/11/SsM-SAP-2016-2025.pdf Accessed 8 September 2026.
- Shen, Y., Zhang, Y. and others. 2024. Genetic diversity and shallow genetic differentiation of the endangered scaly-sided merganser Mergus squamatus. Ecology and Evolution 14: e70011. https://doi.org/10.1002/ece3.70011
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