Smew

The smew (Mergellus albellus) is a small diving duck of the northern Eurasian boreal zone. Adults are about 38–44 cm long, have a wingspan of roughly 56–69 cm and weigh around 500–950 g. Breeding males are unmistakable: mostly white with sharply contrasting black markings around the eye, back and wings. Females and immature birds are greyer, with a chestnut-brown head and white throat.

The narrow, slightly hooked bill is adapted for catching slippery aquatic prey. Smews are smaller and more compact than the larger mergansers, with a rounder head and shorter bill. The species is the only living member of the genus Mergellus. Rare hybrids between smew and common goldeneye (Bucephala clangula) are documented.

During breeding, smews favour clear freshwater lakes, ponds and slow-flowing rivers within boreal forest, especially where mature trees provide nesting cavities close to water. Quiet shorelines, submerged vegetation and shallow feeding areas are important for females with ducklings.

Outside the breeding season the species uses a wider range of open water, including large lakes, reservoirs, broad rivers, estuaries and sheltered coastal bays. Ice conditions strongly affect which sites remain available in winter, so the distribution can shift substantially between cold and mild years.

Small fish dominate much of the winter diet, while aquatic insects, insect larvae, crustaceans and other invertebrates become more important during summer and for young birds. Amphibians and some plant material are also taken. A winter study from Lake IJssel in the Netherlands published in 1979 found that smews concentrated strongly on small fish, especially smelt, and most fish taken were only about 5–6 cm long.

Smews are active visual hunters that pursue prey underwater using the feet for propulsion. In rich winter feeding areas they often chase small pelagic fish in shallow, largely horizontal dives rather than making deep bottom-oriented dives. Observations from Lake IJssel published in 1980 found typical pursuit dives lasting about 15 seconds, with repeated bouts of roughly three dives per minute before the birds paused to rest and preen.

Hunting can become strongly synchronised in flocks. In the same material, nearby birds often disappeared beneath the surface together, and in one densely packed group of about 1,200 smews, 97% of the birds were underwater at the same time for several seconds. Such collective diving does not necessarily mean that the birds deliberately herd fish, but it allows a large flock to exploit the same concentrated shoal simultaneously and can attract other fish-eating birds to the feeding area.

Smews are mainly active by day. Outside the breeding season they often form small flocks and may gather in much larger numbers at particularly rich wintering sites. Breeding birds are more strongly pair-based, while females lead their ducklings to feeding water soon after hatching.

Most smews are migratory. Birds breeding across northern Scandinavia and Russia move south and west as lakes begin to freeze, wintering mainly in temperate Europe and parts of southern and eastern Asia. They return north as open water becomes available in spring.

A study published in 2015, based on counts from 6,883 wetlands in 16 European countries, found that smews shifted their winter distribution north-eastwards between 1990 and 2011 as winters became milder. Numbers increased more strongly inside protected wetlands than outside them, showing that such sites can become especially important as the winter range shifts.

Smews normally nest in cavities in mature trees, often using old woodpecker holes, but readily accept nest boxes. The female usually lays seven to nine eggs, although five to eleven have been recorded, and incubates them alone for about 26–28 days. The ducklings hatch well developed and covered in down and can leave the nest very quickly. They jump from the cavity to the ground and follow the female to water, where they feed for themselves while remaining under her protection. They become capable of flight after about 70 days.

The species can be more flexible than its usual tree-cavity nesting suggests. A study published in 2015 documented at least 15 pairs in Vikna on the central Norwegian coast, far outside typical boreal nesting habitat. Most appeared to nest on the ground rather than in tree cavities. The population shows that smews can adapt both their nesting site and habitat when expanding into new areas.

The smew is classified as least concern on the IUCN Red List. It remains widespread across northern Eurasia, but populations depend on a combination of suitable boreal nesting habitat and high-quality wetlands along migration routes and in winter.

Loss of old cavity-bearing trees, wetland degradation, disturbance, pollution, accidental drowning in fishing gear and predation at nests can affect populations locally. Climate change is already shifting the European winter distribution, while counts show that numbers have developed more positively inside protected wetlands than outside them.