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Smew
Male (top) and female smew, Texel, Netherlands
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Anseriformes
Family: Anatidae
Genus: Mergellus
Species:
M. albellus
Binomial name
Mergellus albellus
Range of M. albellus
  Breeding
  Non-breeding
  Passage
Synonyms
  • Mergus albellus Linnaeus, 1758
  • Mergus minutus Linnaeus, 1758

The smew (Mergellus albellus) is a species of duck and is the only living member of the genus Mergellus. The genus is closely related to Mergus and is sometimes included in it; genetic studies have shown that it is the sister taxon to Mergus and Lophodytes.[2] The smew has hybridised with the common goldeneye (Bucephala clangula).[3]

Taxonomy

[edit]

The smew was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae under the binomial name Mergus albellus.[4] Linnaeus based his account on the description published in 1757 by another Swedish naturalist, Fredrik Hasselqvist, who collected one in Smyrna (now İzmir) in Turkey.[5] Linnaeus specified the type locality as Europe but this was restricted to the Mediterranean near İzmir in Turkey on the basis of Hasselqvist's travels.[6] The smew is now the only living species placed in the genus Mergellus that was introduced in 1840 by the English naturalist Prideaux Selby. The species is monotypic, with no subspecies recognised.[7]

Etymology

[edit]

The genus name Mergellus is a diminutive of Mergus, a Latin word related to mergo, to dive or submerge, used by Pliny for an unspecified diving bird. The specific epithet albellus is a Latin diminutive of albus meaning "white".[8][9]

The term smew has been used since the 17th century and is of uncertain origin. It is believed to be related to the Dutch smient ("wigeon") and the German Schmeiente or Schmünte, "wild duck".[10] It is probably derived from smee, a dialectal term for a wild duck.[11][12]

Description

[edit]
Mergellus albellus female, Northumberland, England

The smew is 38–44 cm (15–17 in) long,[3] with a wingspan of 56–69 cm (22–27 in), and a weight of 450–650 g (16–23 oz).[13]

Adult drake smew, with its 'cracked ice' or 'panda' appearance, is unmistakable, and looks very black-and-white in flight. The females and immature males are grey, with chestnut forehead and crown, white chin and cheeks, and black lores; they are often known as "redhead" smew. Young males moult into adult plumage late in their first winter, when about 8–10 months old; adult males have an eclipse plumage when they become similar to females, but retaining more white in the wing, in late summer and autumn.[3][14] It has oval white wing-patches in flight. The smew's bill has a hooked tip and serrated edges, which help it catch fish when it dives for them.[3]

Distribution

[edit]

This species breeds in the Palearctic in the northern taiga from northern Scandinavia east to Chukotka. It needs trees for breeding. The smew lives on fish-rich lakes and slow rivers. As a migrant, it leaves its breeding areas and winters on sheltered coasts or inland lakes; in Europe in the Baltic Sea, the Black Sea, northern Germany, the Low Countries, and Great Britain (mainly in southeast England but some reaching Scotland); in Asia in the Caspian Sea, in eastern China, Korea, and Japan, and a small number reaching northern India.[3] Vagrants have been recorded in northern Africa (Algeria, Egypt) and North America (Alaska, where regular in the Aleutian Islands, and Canada).[3]

Ecology

[edit]

On lakes it prefers areas around the edges, often under small trees. The smew breeds in May and lays 7–11 cream-coloured eggs, incubated by the female for 26–28 days. Ducklings leave the nest soon after hatching and learn to fly within about 10 weeks.[13]

It nests in tree holes, such as old woodpecker nests. It is a shy bird and flushes easily when disturbed.

The smew is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies. It is not considered threatened on the IUCN Red List, though its population is decreasing.[1]

Fossil history

[edit]

Subfossils from this species have been found in the Early Pleistocene of West Runton, England.[15]

[edit]
  • Male taking off, Helsinki, Finland
    Male taking off, Helsinki, Finland
  • Male, Japan
    Male, Japan
  • Male in flight, Japan
    Male in flight, Japan
  • Female, England
    Female, England
  • ID composite
    ID composite
  • Smew eggs, from Suomussalmi, Finland, at the Muséum de Toulouse
    Smew eggs, from Suomussalmi, Finland, at the Muséum de Toulouse

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Smew

View on Grokipedia
from Grokipedia
The Smew (Mergellus albellus) is a small, migratory diving duck belonging to the family , characterized by its compact size (approximately 35–41 cm in length) and distinctive in . Males feature a striking black-and-white pattern, with a snow-white body, black face mask, back, and wings, often topped by a bushy crest, while females are predominantly with a rusty-brown head, white cheek patch, and shorter bill compared to other mergansers. This is a full migrant, breeding in the forests of and before wintering in temperate zones across and occasionally vagrating to . Native to , the Smew's breeding range spans vast boreal landscapes from through to eastern , where it favors clear, oligotrophic lakes and rivers surrounded by coniferous forests for nesting in tree cavities, often those excavated by woodpeckers like the . During winter, it shifts to larger, more open freshwater bodies such as reservoirs, coastal lagoons, and sheltered estuaries in regions like the , , and southern Asia, where it forms gregarious flocks outside the breeding season. As a diving duck, it employs rapid wingbeats in direct flight and forages by submerging to pursue prey, primarily small and aquatic in winter, shifting to , crustaceans, and amphibians during the breeding period when feeding young. Breeding occurs from May to July in the northern , with females laying 8–11 eggs in secondary cavities or artificial nestboxes; the is monogamous for the season, and ducklings are precocial, leaving the nest shortly after to follow the . Generally shy and vocalizing with simple rattle calls, Smews are diurnally active and often rest on inland waters during migration. The global is estimated at 58,700–144,000 mature individuals, with a stable but unknown trend, classifying it as Least Concern on the despite localized threats from habitat loss, , and climate change impacts on boreal wetlands.

Taxonomy and evolution

Etymology

The scientific name of the smew is Mergellus albellus. The genus name Mergellus is a form of the Latin Mergus, which refers to a or waterfowl, derived from the verb mergō meaning "to dive" or "to plunge". The species albellus comes from Latin albellus, meaning "whitish" or "somewhat ," a of albus (""), alluding to the predominantly of the adult male. The common English name "smew" has been in use since the but is of uncertain origin. It is possibly related to the Dutch smient (meaning , a type of ) or the German Schmeiente or Schmünte (referring to a wild ), though the exact derivation remains obscure. In English, the name is pronounced /smiːuː/.

Classification

The smew (Mergellus albellus) belongs to the family , the ducks, geese, and swans, within the order . It is classified in the subfamily (true ducks) and the tribe , which encompasses the sea ducks or diving ducks adapted for underwater foraging. The species is the sole extant member of the monotypic genus Mergellus, with no recognized , rendering M. albellus a . Phylogenetic analyses, including cladistic studies of morphological characters, position Mergellus as closely allied to the genus , which includes species like the (M. merganser). This relationship is supported by shared traits such as serrated bills for catching fish (sawbills) and overall cranial and skeletal morphology. Molecular evidence from sequences further corroborates this affinity, placing the smew within the sawbill of . Studies from the early 2000s using mtDNA control regions and other genes estimate the divergence of the Mergellus lineage from Mergus at approximately 2–3 million years ago, reflecting a relatively recent split within the . The genus name Mergellus is a form of Mergus, highlighting this close taxonomic link.

Fossil record

The earliest fossils attributable to the smew, identified as Mergellus albellus, date to the early Middle Pleistocene, approximately 0.7 million years ago (Cromerian stage, ~0.8–0.5 Ma), and were recovered from the West Runton Freshwater Bed within the Cromer Forest Bed Formation in , . These remains, including a proximal end of a left and distal end of a right , represent the oldest direct evidence of the and indicate that smew-like ducks inhabited northern European freshwater environments during this period, a time of cooling climates and expanding woodlands. A tentative assignment of a to Mergellus from the Middle (~13.7–11.1 Ma), Upper Badenian stage, at Rákóczi Chapel, Mátraszőlős, , suggests possible older occurrences, but this is not definitive. In contrast, closely related genera within the sawbill ducks, such as Mergus, are known from much earlier Miocene deposits (23–5 million years ago), including M. miscellus from the Calvert Formation in , , around 14 million years ago. These records suggest that the broader sawbill (Mergini) lineage originated during the late Tertiary, with Mergellus diverging later and achieving a distinct Eurasian distribution by the Pleistocene. The also includes an extinct , Mergellus mochanovi, known from the (~14,000–12,000 years ago) in Dyuktai Cave, Yakutia, northeastern . Fossils include a right tibiotarsus (), partial left carpometacarpus, and a complete left carpometacarpus; this was smaller than the extant smew and shows some similarities to the (Bucephala albeola). The persistence of the smew lineage through multiple Pleistocene glaciations is implied by these fossils and later records, pointing to survival in southern Eurasian refugia where boreal forest habitats remained viable amid fluctuations. This evolutionary continuity underscores the adaptability of Mergellus to cold-temperate ecosystems, closely aligning it with the modern in phylogenetic and ecological terms.

Description

Size and structure

The smew (Mergellus albellus) is a small diving measuring 35–44 cm in length, with a of 55–69 cm and a weight of 510–935 g. Its compact body features a short and rounded head, contributing to a streamlined form suited for underwater movement, while the fully webbed feet provide propulsion during dives. The bill is short and slender, ending in a hooked tip with serrated edges characteristic of sawbills, enabling a secure grip on slippery prey such as ; it is narrower overall than those of larger merganser . Sexual size dimorphism is minimal, with males averaging about 15% heavier than females.

Plumage

The smew (Mergellus albellus) exhibits striking in its , with the adult male in breeding condition displaying a predominantly white body accented by bold black markings. The head features a distinctive black mask extending from the lores around the eyes, paired with a white elongated crest bordered by black feathers on the , while the back, mantle, and scapulars are black, creating a "panda-like" or "cracked ice" pattern due to fine black vermiculations on the white flanks and underparts. The wings are largely black with a prominent white patch on the secondary coverts, visible during flight, and the two narrow black lines on each side of the breast further enhance the contrasting pied appearance. In contrast, the adult smew, often called a "redhead," has a more subdued grey-brown body suited for , with mottled grey upperparts, breast, and flanks that darken toward the rear. The head and are or rufous-brown, sharply set off by bold cheeks, , and a patch under the eye, while the wings show less than the male's, with grey greater coverts and a smaller white speculum. Juveniles resemble the but are duller overall, with brownish-grey tones, darker lores lacking the male's , and less distinct facial patches; young males transition to adult in their first winter at around 8-10 months old. During the post-breeding , the adult male molts into a resembling the female's grey-brown overall but retains diagnostic black upperparts and a white patch on the median wing coverts for identification. This annual wing molt, typical of the tribe, involves simultaneous replacement of all shortly after breeding, rendering males flightless for 3-4 weeks as they gather in flocks near breeding sites to recover.

Distribution and habitat

Breeding distribution

The smew (Mergellus albellus) breeds across the taiga zone of the northern Palearctic, primarily in boreal forests extending from eastward to the , including regions up to Chukotka and Kamchatka. This range encompasses dense coniferous and mixed woodlands associated with freshwater systems in and . In , the breeding population is estimated at 10,800–17,000 pairs (as of 2021), representing a minor portion of the global total, with the highest densities in (8,000–10,000 pairs, comprising about 66% of the European population) and (2,000–5,500 pairs, or 26%). The overall global population is estimated at 58,700–144,000 mature individuals (as of 2021), considered stable, with the core and densest breeding concentrations occurring in Siberian taiga regions of Asian . Smews typically arrive at breeding sites from to early May, initiating nesting in mid-May amid remote boreal lakes and slow-flowing rivers rich in ; they preferentially select cavities, such as those excavated by woodpeckers, for nesting sites.

Non-breeding distribution

During the non-breeding season, the smew (Mergellus albellus) winters primarily in temperate regions across , favoring ice-free coastal and inland waters. In , key wintering areas include the coasts of the Baltic and Seas, as well as estuaries and sheltered bays in the , , and other southern European countries such as , , and . Further east, the species occurs along the and extends through to eastern , Korea, , and occasionally northern . The global winter population is estimated at 88,000–216,000 individuals (as of 2023), with Europe's non-breeding contingent comprising 48,700–166,000 birds, often forming flocks of 10–100 on coastal lagoons, large lakes, or reservoirs, though congregations exceeding 10,000 have been recorded at prime sites. Vagrant occurrences are noted outside the core range, particularly in , where the species appears irregularly in the of and casually elsewhere in the and . Rare sightings persist in , including the and parts of the , where numbers have fluctuated but shown a general decline in the west amid milder continental winters. is influencing distribution, with wintering populations in north-east increasing from 6% to about one-third of the total over two decades (1990s–2010s), potentially expanding ranges northward and into protected areas.

Habitat preferences

The smew (Mergellus albellus) exhibits distinct habitat preferences that align with its diving behavior and cavity-nesting requirements, favoring clear, nutrient-poor waters surrounded by mature year-round. During the breeding season, it selects boreal coniferous and mixed forests adjacent to shallow freshwater bodies, such as oligotrophic lakes, pools, lakes, and backwaters of slow-flowing rivers, typically with depths up to 4 m (maximum 6 m). These sites provide abundant and prey in clear waters, while the surrounding dense woodland—often featuring mature broadleaved trees like oaks, willows, and aspens—offers natural cavities or old holes (frequently excavated by black woodpeckers) up to 10 m above ground for nesting. The species avoids open, treeless habitats, relying instead on forested edges for security due to its shy disposition. In winter, smews shift to ice-free, sheltered aquatic environments that support their prey of small , aquatic , and crustaceans, including large freshwater lakes, reservoirs, brackish coastal lagoons, estuaries, and floodplains with clear water. Preferred sites feature protective cover such as vegetated edges or overhanging trees, allowing the birds to in relative while resting on smaller or bays during passage. This preference for sheltered, vegetated margins over exposed open seas underscores their need for low-disturbance conditions in temperate to mild coastal or inland settings. Overall, smews require oligotrophic conditions in breeding waters for sufficient prey productivity, complemented by emergent vegetation and forested peripheries that provide both foraging access and escape cover, reflecting their elusive nature across seasons.

Behavior

Breeding behavior

The smew is seasonally monogamous, with pair bonds typically forming in late winter, often from late onward, during communal displays on wintering grounds. Males perform elaborate behaviors to attract s, including head-throwing, where the raises and tosses its head backward, and wing-whirring, producing a distinctive fluttering while flapping its wings rapidly. These displays often occur in small groups of 2–7 s attending 1–2 s, with additional actions such as circling the female, head-shaking, and the "pouting" posture, where the moves its horizontal head back and forth over its mantle. Pairs migrate to breeding areas together, maintaining the bond for the season until the female begins incubation. Breeding occurs from mid-May onward in the northern parts of the range, with smews being secondary cavity nesters that select natural tree holes or abandoned nests, often those of the , up to 10 m above ground in mature broadleaf trees such as , , or aspen near freshwater lakes and rivers. The female lines the cavity with down and lays a of 7–9 eggs (ranging from 5–11), which she incubates alone for 26–28 days; the male abandons the female shortly after completion to moult in flocks nearby. Eggs are cream-colored, measuring approximately 53 × 38 mm and weighing about 42 g. Ducklings are precocial, hatching covered in down and leaving the nest shortly after to follow the female to water, where she broods and protects them, leading foraging efforts while providing warmth, especially in the first few weeks. They fledge after approximately 70 days, becoming independent soon thereafter, with the female eventually abandoning the brood. Males do not participate in beyond the pre-incubation period. Breeding success varies, with average brood sizes of about 5.6 ducklings (ranging 2–13), implying and fledging rates of 50–70% in favorable conditions, heavily influenced by the availability of suitable nesting cavities; artificial nest boxes can enhance success in areas with limited natural holes.

Feeding behavior

The smew (Mergellus albellus) employs a diving technique to pursue and capture prey, propelling itself underwater with powerful wings and feet after initially scanning the surface with its head immersed. These dives typically reach depths of 1–4 , allowing access to benthic habitats where food is abundant. The ' diet is carnivorous and opportunistic, comprising mainly aquatic such as insect larvae and adult insects (e.g., water beetles), small like minnows, crustaceans, and mollusks, with lesser amounts of amphibians and incidental plant material including seeds and roots. The smew's slender bill, featuring serrated edges and a hooked tip, facilitates gripping slippery prey such as during underwater chases. is diurnal and occurs in small flocks or solitarily, with individuals spending much of the day actively diving in shallow, vegetated waters to meet energetic demands. Dietary composition varies seasonally to align with prey availability and reproductive needs. During the breeding season, the focus shifts toward protein-rich and their larvae, which are essential for provisioning ducklings and promoting rapid growth. In winter, small dominate the intake, reflecting the abundance of schooling prey in non-breeding habitats.

Migration and movements

The Smew (Mergellus albellus) is highly migratory, departing its breeding grounds in the northern taiga of and in late summer and autumn to reach temperate wintering areas across western and , the , the Black Sea region, , the , and eastern . Departure from breeding areas commences in early , with the main southward passage completed by early ; birds typically arrive at wintering sites from to . The return migration northward begins in March, becoming noticeable that month, with most individuals arriving back at breeding grounds in April or early May. Smews travel overland through Europe and Asia, relying on inland lakes and waterbodies as key stopover sites for resting and feeding during passage. During migration, smews form loose groups or flocks, typically numbering fewer than 100 individuals, though larger gatherings of up to 10,000 have been recorded in some contexts. Vagrancy occurs occasionally due to storms or navigational errors, with records in North America including regular small numbers in the Aleutian Islands of Alaska and rare sightings elsewhere on the continent. Climate warming has influenced the ' movements, driving a northeastward shift in its wintering distribution across as milder conditions allow occupation of more northerly sites.

Conservation

Status

The smew (Mergellus albellus) is classified as Least Concern on the , with the most recent assessment in 2025 confirming that it does not meet the criteria for higher threat categories due to its large range and stable populations in key areas. Global population estimates indicate 58,700–144,000 mature individuals, though comprehensive worldwide counts remain challenging owing to the ' vast boreal breeding range across . In , the breeding population is estimated at 10,800–17,000 pairs (equivalent to 21,700–34,000 mature individuals), while wintering numbers reach 48,700–166,000 individuals. The bulk of the global population occurs in , where breeding densities are highest in and eastern , contributing to the overall estimate but with less precise quantification due to remote habitats. Population trends are considered stable across much of the range, with Europe's overall breeding numbers showing relative stability over the past three generations (approximately 12.5 years), despite local variations such as declines in and increases in and . In , available data suggest stability, with no evidence of widespread declines. Globally, trends are unknown but not believed to approach reduction thresholds for threatened status. The smew has been protected under the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) since its entry into force in 1996, listed in Appendix II for populations in north-west and central Europe, north-east Europe/, and western Siberia/south-west , requiring international cooperation for conservation. It is also listed under CMS Appendix II and Bern Convention Appendix II. Monitoring efforts include systematic counts through the International Waterbird Census for wintering populations and national breeding surveys in key areas, often incorporating aerial methods to cover extensive boreal forests in Europe and Asia. These approaches provide essential data for tracking distribution and abundance in remote habitats.

Threats

The smew (Mergellus albellus) faces primary threats from habitat loss, driven by logging in boreal forests that diminishes mature trees and the natural cavities essential for nesting. Tree cover within its mapped range has declined by 8.8% over the past three generations, exacerbating the scarcity of suitable breeding sites. Wetland drainage for agriculture and development further degrades foraging and breeding habitats in river valleys and forested wetlands. Pollution represents another key risk, particularly oil spills in coastal wintering areas where smews congregate in large numbers, leading to direct mortality and contamination of food sources. In parts of , hunting pressure during migration and overwintering periods contributes to population declines, with birds often targeted for sport and trade. Predation by invasive (Neovison vison) poses a threat to breeding populations in , particularly where natural cavities are limited. Avian influenza outbreaks have also been identified as a potential risk to waterbirds like the smew. Climate change poses an emerging threat by altering boreal ecosystems through shifts in temperature, precipitation, and hydrology, which may disrupt breeding ranges and suitability. These factors, combined with habitat degradation, have been linked to localized declines in vulnerable regions. Conservation efforts include the designation of protected areas, such as Special Protection Areas (SPAs) under the EU Birds Directive, which have supported population growth rates doubling inside these sites compared to surrounding areas over 25 years. In Scandinavian countries, artificial nest boxes are deployed to compensate for lost natural cavities, and smews readily adopt them for breeding. International monitoring through the International Waterbird Census, coordinated by , tracks population trends and informs targeted interventions. These measures have contributed to increases in wintering populations in northeast , rising from 6% to about one-third of the total over two decades.

References

  1. https://www.audubon.org/field-guide/bird/smew
  2. https://birdsoftheworld.org/bow/species/smew/cur/introduction
  3. https://ebird.org/species/smew
  4. https://datazone.birdlife.org/species/factsheet/smew-mergellus-albellus
  5. https://www.merriam-webster.com/dictionary/Mergus
  6. https://en.wiktionary.org/wiki/mergus
  7. https://uk.inaturalist.org/taxa/7192-Mergellus-albellus
  8. https://animalia.bio/smew
  9. https://www.researchgate.net/publication/267398965_Ducks_Geese_and_Swans_of_the_World_Tribe_Mergini_Sea_Ducks
  10. https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.104708/Mergellus_albellus/
  11. https://digitalcommons.usf.edu/cgi/viewcontent.cgi?article=13574&context=condor
  12. https://www.sciencedirect.com/science/article/abs/pii/S1055790302000192
  13. https://birdfact.com/birds/smew
  14. https://www.wildlifetrusts.org/wildlife-explorer/birds/waterfowl/smew
  15. http://www.arthurgrosset.com/europebirds/smew.html
  16. https://worldspecies.org/ntaxa/829427
  17. http://www.oiseaux-birds.com/card-smew.html
  18. https://www.birdweb.org/birdweb/bird/smew
  19. https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/06-1751.1
  20. https://birdlifedata.blob.core.windows.net/sub-global/497_mergellus_albellus.pdf
  21. https://www.bto.org/learn/about-birds/birdfacts/smew
  22. https://bcrarebirdrecords.ca/species/smew
  23. https://global.oup.com/academic/product/ducks-geese-and-swans-9780198610083
  24. https://www.oiseaux-birds.com/card-smew.html
  25. https://doi.org/10.1111/ddi.12300
  26. https://www.birdwords.co.uk/home-v/bird-factsheets/smew-mergellus-albellus/
  27. https://glenlivet-wildlife.co.uk/birds/smew/
  28. https://www.allevamentopoggiodiponte.com/our-breeding/ducks/sea-ducks/smew
  29. https://doi.org/10.2173/bow.smew.01.1
  30. http://migrationatlas.org/node/1561
  31. https://onlinelibrary.wiley.com/doi/10.1111/ddi.12300
  32. https://www.iucnredlist.org/species/22680465/166210547
  33. https://www.unep-aewa.org/sites/default/files/instrument/aewa_agreement_text_2023-2025_mop8.pdf
  34. https://core.ac.uk/download/pdf/225903240.pdf
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