Common Eider

The common eider, also called St. Cuthbert's duck or Cuddy's duck, is a large sea-duck that is distributed over the northern coasts of Europe, North America and eastern Siberia. It breeds in Arctic and some northern temperate regions, but winters somewhat farther south in temperate zones, when it can form large flocks on coastal waters. It can fly at speeds up to 113 km/h (70 mph).

Common Eiders are diurnal, spending daylight hours foraging and resting in flocks on open water or along rocky coasts. They are skilled divers, using their large webbed feet to propel themselves underwater in search of prey, mainly blue mussels (Mytilus edulis), sea urchins, crustaceans, and occasionally small fish. Eiders often feed in synchrony, forming loose groups that dive and surface together. Social structure is complex, especially during breeding, with females forming 'crèches'—groups where multiple hens jointly care for ducklings, increasing survival rates. Outside the breeding season, both sexes are highly social, forming large, mixed flocks. Eiders are relatively sedentary, showing strong site fidelity to both breeding and wintering grounds. Daily routines include preening, bathing, and resting on water or exposed rocks.

Breeding occurs from late May to July, depending on latitude. Males perform elaborate courtship displays, including head-throwing and vocalizations. Eiders are mostly monogamous within a breeding season, though pair bonds may not persist year to year. Females select nest sites on coastal islands, often in dense colonies, and line their nests with down plucked from their own breasts. Clutch size ranges from 3 to 6 pale olive eggs. Incubation lasts 25–28 days and is performed solely by the female, who fasts during this period. Ducklings are precocial, leaving the nest within 24 hours of hatching and heading to water, where they are cared for by the mother and sometimes by multiple females in communal broods. Fledging occurs at 65–75 days, but young eiders remain vulnerable to predation and harsh weather.

Common Eiders exhibit several adaptations for cold, marine environments. Their dense, waterproof feathers and thick layer of eiderdown provide exceptional insulation, allowing them to remain in frigid waters. Salt glands above the eyes excrete excess salt, enabling them to drink seawater. Their wedge-shaped bills are specialized for prying open mollusks, and their strong, webbed feet aid in efficient diving. Behavioral adaptations include colonial nesting for predator defense and communal brood-rearing to increase duckling survival. Eiders are also capable of reducing metabolic rates during fasting periods, such as incubation.

The Common Eider has long been valued for its down, which is harvested sustainably in some regions (notably Iceland and Norway) without harming the birds. Eiderdown is considered the finest natural insulator and has been used in bedding for centuries. The species is associated with St. Cuthbert, an Anglo-Saxon saint, leading to the nickname 'Cuddy's duck.' Eiders feature in northern European folklore and have been depicted in art and literature as symbols of coastal wilderness and resilience.

Recent research has focused on the effects of climate change on eider breeding phenology and chick survival, as well as the impact of marine pollution on health and reproductive success. Genetic studies have clarified subspecies relationships and migration patterns, revealing limited gene flow between some populations. Ongoing studies are examining the sustainability of down harvesting and the role of communal brood-rearing in chick survival. Notably, eiders have been used as model organisms for studying the energetics of fasting and thermoregulation in birds.

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The Common Eider is currently classified as Least Concern on the IUCN Red List.

Major threats to Common Eiders include oil spills, entanglement in fishing gear, overharvesting of eggs and down, and predation by introduced mammals (e.g., foxes, rats) on breeding islands. Climate change poses additional risks by altering prey availability and increasing storm frequency, which can flood nests. While global populations are currently classified as Near Threatened by the IUCN, some regional populations have experienced significant declines due to habitat disturbance, pollution, and disease outbreaks (e.g., avian cholera). Conservation challenges include protecting breeding colonies, regulating down collection, and mitigating human disturbance.