Scaly-sided merganser (Mergus squamatus)

GenusMergus (1)
SizeSize: 52 – 58 cm (2)

Classified as Endangered (EN) on the IUCN Red List 2007 (1), and listed on Appendix II of the Convention on Migratory Species (CMS) (3).

This distinctive species earns its unusual name for the white feathers of its flanks, which are boldly edged and shafted with grey-black, giving a scaled appearance (4). Mature males have a creamy-white breast and underparts, greyish-black upperparts, and a glossy greenish-black head and neck with a long, droopy, shaggy crest (2). By contrast, adult females have a warm buffish head and neck and only a wispy crest (2). Both sexes have long, serrated orange-red bills and similarly coloured legs and feet (4).

The scaly-sided merganser breeds in south-east Russia, North Korea and north-east China. Some birds spend the winter in south-east Russia, but most are thought to winter in central and southern China. Small numbers winter in Japan, North Korea, South Korea and Taiwan (China), and a handful of records exist from Myanmar, Thailand and northern Vietnam (5).

Breeding occurs along the middle reaches of well-forested, fast-flowing mountain rivers and rapid streams, below around 900 metres above sea level, where the bird is largely confined to primary forests with an abundance of potential nest holes (2) (6). The non-breeding season is spent on larger lakes and more sluggish rivers and lagoons (6).

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 (5). 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 (7). 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 (5).

The scaly-sided merganser usually forages in small groups of up to three birds and feeds on small fish as well as insect larvae, shrimps, crayfish and beetles, taken from the river (5).

This rare, stunning bird has undergone significant declines as a result of habitat destruction, persecution and disturbance. Populations suffered badly in Russia in the 1960s and 1970s, when a period of intense economic development led to extensive habitat destruction and alteration. Thankfully, large scale deforestation in river valleys has since been banned and the rapid decline in this species’ population was halted (5). Nevertheless, a variety of other threats still affect the scaly-sided merganser in Russia, including forest fires, illegal hunting, drowning in fishermens' nets, disturbance from motor boats during the breeding season, river pollution and natural predators (2) (5). In China, breeding populations continue to decline rapidly in the face of deforestation, illegal hunting, human disturbance and the use of poisons and/or explosives for fishing, and have disappeared from much of their former range (2).

The scaly-sided merganser is a Nationally Protected Species (First Class) in China, a protected species (category 1) in North Korea and a protected species in South Korea (5). This bird can also be found in a number of protected areas in both its breeding and wintering range, notably Sikhote-Alin' State Biosphere Reserve in Russia, and Changbai Shan Nature Reserve in China (2) (5). A number of studies of this species have been conducted, including research that has shown females readily use artificial nest boxes, with several returning to breed in the same boxes in subsequent years. Over 130 artificial nest boxes have been erected alongside Russian rivers (8), which not only increase the availability of nest sites and therefore breeding capacity, but also provide a valuable opportunity to study the breeding biology of this rare duck.

For more information on the scaly-sided merganser see:

Information authenticated (02/05/07) by Dr Baz Hughes, Head of Species Conservation, Wildfowl and Wetlands Trust, Slimbridge, UK.

  1. IUCN Red List (November, 2011)