Scaly-sided Merganser (Mergus squamatus)

Scaly-sided Merganser

[order] ANSERIFORMES | [family] Anatidae | [latin] Mergus squamatus | [authority] Gould, 1864 | [UK] Scaly-sided Merganser | [FR] Harle ecaille | [DE] Schuppensager | [ES] Serreta China | [NL] Chinese Zaagbek


Monotypic species


Mergus is the genus of the typical mergansers, fish-eating ducks in the seaduck subfamily (Merginae). The Hooded Merganser, often termed Mergus cucullatus, is not of this genus but closely related. The other “aberrant” merganser, the Smew (Mergellus albellus), is phylogenetically closer to goldeneyes (Bucephala). Although they are seaducks, most of the mergansers prefer riverine habitats, with only the Red-breasted Merganser being common at sea. These large fish-eaters typically have black-and-white, brown and/or green hues in their plumage, and most have shomewhat shaggy crests. All have serrated edges to their long and thin bills that help them grip their prey. Along with the Smew and Hooded Merganser, they are therefore often known as “sawbills”. The goldeneyes, on the other hand, feed mainly on mollusks, and therefore have a more typical duck-bill. They are also classified as “divers” because they go completely under-water in looking for food. In other traits, however, the genera Mergus, Lophodytes, Mergellus, and Bucephala are very similar; uniquely among all Anseriformes, they do not have notches at the hind margin of their sternum, but holes surrounded by bone.

Physical charateristics

Striking merganser with shaggy crest and scaled flanks. Adult male has black, glossed green head and neck with long crest. Creamy-white lower foreneck, breast and central underparts. Whitish flanks, ventral region, and rump with grey scaling. Blackish mantle, hindneck and scapulars. Mostly white innerwing. Adult female has warm buffish head and neck with dusky lores and wispy crest. Whitish breast and central underparts. Male Red-breasted Merganser M. serrator has white collar and rufous breast and lacks heavy scaling on flanks. Female also lacks scaling.

wingspan min.: 0 cm wingspan max.: 0 cm
size min.: 52 cm size max.: 62 cm
incubation min.: 31 days incubation max.: 35 days
fledging min.: 55 days fledging max.: 65 days
broods: 1   eggs min.: 4  
      eggs max.: 12  


Eurasia : East. Mergus squamatus breeds in Khabarovsk, Amur, the Jewish Autonomous Region and Primorye in south-east Russia, North Korea and Heilongjiang, Jilin and Inner Mongolia in north-east China. Some birds winter in south-east Russia and North Korea, but most winter in central and southern China (the majority of wintering flocks found on the lower branch of the Yangtze River), with small numbers in Japan, South Korea, Taiwan (China), Myanmar and Thailand.


It breeds below c.900 m in mountainous areas, along rivers with tall riverine forest, mainly within the temperate conifer-broadleaf forest zone. It is largely confined to primary forests, with an abundance of potential nest-holes. During a study on Russian breeding grounds river size, mountain slope, human population, estimated forest cover and water clarity all failed to explain the observed distribution, but the species showed a marked preference for the middle reaches of rivers. On passage and in winter it feeds along large rivers. Flocks of up to 20 individuals have been noted on passage or in winter


These birds return to breeding sites in late March to early April. Their breeding system is a mixture of monogamy and polygamy, and they establish breeding territories along a stretch of river by mid-April. Unusually amongst ducks, breeding trios of one male and two females are sometimes formed; breeding trios like these comprise up to 20 percent of breeding populations in Far East Russia. Clutches of four to twelve eggs are laid from the second half of April and throughout much of May. By early June, males leave the breeding grounds, while the females remain to incubate their eggs for 31 to 35 days. Nests are established in tree holes up to 18 metres above the ground, lined with down. Normally one clutch is laid per year, but if the first is destroyed a replacement may be laid. Broods hatch from May to June and most chicks fledge in the last ten days of August, at around eight weeks of age. In September and October, the birds migrate for the winter.

Feeding habits

The food of M. squamatus consists of aquatic arthropods and small or young fish. Stonefly (Plecoptera) and Phryganeidae giant caddisfly larvae may constitute the bulk of its diet when available. Beetles and crustaceans are eaten less regularly, though the latter may be more important in autumn. As aquatic insect larvae hatch in the course of the summer, fish become more prominent in the diet. Favorite fish species include the Dojo Loach (Misgurnus anguillicaudatus) and the lenok Brachymystax lenok. More rarely eaten are such species as the lamprey Eudontomyzon morii, the sculpin Mesocottus haitej, or the Arctic Grayling (Thymallus arcticus). Thus, they are opportunistic feeders; regarding fish, they will probably eat any species that has the correct elongated shape and small size.

Video Scaly-sided Merganser


copyright: Josep del Hoyo


This species has a very small population which is suspected to be undergoing a continuing and rapid decline as a result of habitat loss, illegal hunting and disturbance. It is therefore listed as Endangered.
In the 1960s and 1970s, its decline in Russia coincided with economic development of the taiga. Primary forests in the valleys of all large rivers were greatly altered, but large-scale deforestation in river valleys is now prohibited, however the new Russian Forest Codex (2007) requires a water protection zone (no deforestation) of only 100 m for large rivers (50 m on each side), and 50 m (25 m each side) for rivers shorter that 100 km, which is likely to significantly reduce suitable breeding habitat for the merganser, which nests up to 150 m from the river. Logging of river sources and adjacent slopes has led to reduced spring water levels and changes in fish abundance; since logging began on the Avvakumovka River in 2004 spring water levels and merganser populations have undergone continuous declines. Other major threats within the breeding range include illegal hunting, drowning in fishers’ nets (a major cause of mortality at Russian breeding sites in 2003-2007), disturbance from motor boats during the breeding season, river pollution and natural predators. Increased hunting of waterfowl for sport together with poor regulation of the spring hunting season (which is intended to coincide with passage migration and avoid targeting locally breeding birds) is a significant and increasing threat; large numbers were reportedly shot in the Kievka River basin, southern Primorye, in spring 2008. Threats in its Chinese breeding range include dam construction, deforestation, illegal hunting, human disturbance and the use of poisons and/or explosives for fishing
Scaly-sided Merganser status Endangered


M. squamatus are migratory, wintering in central and southern China, with small numbers in Japan, South Korea, Taiwan, northern Vietnam, Myanmar and Thailand. They arrive on the breeding grounds as soon as winter is over, in March, and leave when the first cold nights come in late October

Distribution map

Scaly-sided Merganser distribution range map

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