The common eider (Somateria mollissima) is a large sea duck that breeds in the northern coastal regions of Europe, Asia, and North America. The females build nests on the ground in sheltered locations and line them with their down feathers to incubate the eggs. Common eiders are migratory birds and their nesting habits vary across different parts of their range.
Nesting habitats
Common eiders nest in a variety of coastal habitats in the northern hemisphere during the spring and summer months. Their nesting habitats include:
Islands
– Small, rocky islands off the coast provide protected nesting sites safe from land predators. Islands in northern Canada, Iceland, Norway, Scotland, Alaska, and Russia are used.
Coastal tundra
– Open, treeless tundra along ocean coasts offer flat, open terrain for nesting. Areas along the Beaufort Sea, Hudson Bay, and Baffin Island are used.
Deltas and estuaries
– Productive intertidal areas at the mouths of rivers provide food sources. Key sites are the Mackenzie River delta and the Lena River delta.
Beaches
– Secluded sand beaches along shorelines offer concealment for nests. Beaches in Iceland, Finland, Scotland, Maine, and Scandinavia are occupied.
Dunes and marshes
– Coastal dunes and marshes offer protection from wind and tides. Locations in Scotland, England, Germany, and Russia are used.
Tundra lakes and ponds
– Lakeshores and pond edges in tundra provide safety from foxes. Sites in Russia, Alaska, Canada, and Svalbard are used.
Nest locations
Female common eiders search for nest sites that provide:
– Concealment from predators in vegetation or rocky terrain
– Insulation from cold ground using dead vegetation or lichens
– Access to feeding areas on ocean shorelines
– Protection from flooding by tides or storms
– Safety from human disturbance
Specific nesting sites include:
Rocky crevices and cavities
– Females nest in rocky holes and crevices on islands and shorelines. These sites protect eggs but can make them vulnerable to avian predators.
Driftwood and debris
– Logs, driftwood, and vegetation debris above the high tide line are used. These offer cover and warmth.
Dunes and hollows
– Hollows on the leeward side of dunes or earth banks provide concealment from winds.
Low shrubs and grass
– Dense patches of low vegetation conceal nests from aerial predators. Willow thickets, beach grass, and low shrubs are used.
Muskox skulls
– The empty skulls of muskoxen offer protected nesting cavities on tundra. Eiders line them with down feathers.
Nest characteristics
The eider’s nest is a scraped out depression or bowl lined with vegetation, lichens, and down feathers. Nests exhibit the following traits:
– Diameter: 20-25 cm
– Depth: 5-10 cm
– Lined with down feathers plucked from the female’s breast
– Insulated with dry grasses, mosses, lichens
– Woven into surrounding vegetation for concealment
– Reused for multiple years
– Defended aggressively by nesting females
The soft down provides superb insulation for the eggs, keeping them up to 20°C warmer than ambient temperatures. The down lining also allows eggs to survive temporary flooding by tides.
Clutch size and incubation
– The typical clutch size is 3-6 eggs (70 x 47 mm size)
– The female incubates them for about 26 days
– During incubation, females leave the nest only briefly 1-2 times daily to feed
– Males depart shortly after mating and do not participate in rearing young
The incubating females sit tightly on the nest when approached and may appear injured to deter predators. Nesting eiders have effective camouflage and are well hidden in their nest sites.
Nesting schedule
Nest initiation dates vary across the common eider’s breeding range:
| Location | Nest Initiation |
|---|---|
| Beaufort Sea, Canada | Early June |
| Hudson Bay, Canada | Late May to early June |
| Maine, USA | Early to mid May |
| Scotland, UK | Late April to early June |
| Norway | April to June |
| Finland | May to June |
| Iceland | Early to mid June |
| Russia | Late May to July |
Later nesting coincides with delayed ice breakup at higher latitudes. Most nests are initiated in May and June to allow time for the ducklings to hatch and grow before autumn migration.
Nest density
At prime nesting areas, common eider nests may occur at high densities of up to 110-150 nests/hectare. This facilitates group defense against predators. However, nests are typically dispersed at much lower densities across the landscape to provide adequate food resources. Reported densities include:
– Maine, USA – 6-13 nests/hectare (coastal islands)
– Iceland – 3-30 nests/hectare
– Finland – 2-15 nests/hectare
– Norway – up to 70 nests/hectare (island colonies)
– Russia – 0.2-2 nests/ hectare (coastal tundra)
The availability of safe nesting habitat limits populations in many areas. Eiders exhibit high fidelity to ancestral nesting islands and coastal sites.
Non-breeding females
Not all adult female common eiders nest each year. Up to 40-50% may skip breeding under poor habitat or food conditions. These non-breeders often congregate together in large rafts offshore of nesting islands. The proportion breeding annually influences population counts.
Nest predation
Despite their secretive and well-insulated nests, common eiders suffer substantial losses to predators. Documented nest predators include:
– Arctic foxes – Major predator of ground nests in tundra
– Red foxes – Threat on northern islands
– Raccoons – Invade island colonies on Atlantic coast
– Ravens – Corvids plunder eggs and young
– Herring gulls – Abundant gull colonies produce egg thieves
– Polar bears – Opportunistically predate nests and females in the high Arctic
– Weasels, mink, and others – Take eggs and ducklings
Mammals are unable to reach most cliff and crevice sites used on rugged coasts and islands. Aerial predators take a higher toll in open tundra habitats. Predation limits nest success in many regions.
Breeding site fidelity
Female common eiders exhibit strong fidelity to ancestral breeding locales and nest sites between years. Many return to natal areas or previous nest sites where they have bred successfully, bypassing poorer-quality habitats.
Factors driving nest site fidelity include:
– Familiarity with resources
– Prior nest success
– Kin relationships with other nesters
– Costs and risks of finding new sites
This loyalty concentrates populations and facilitates monitoring. It also makes eiders vulnerable to habitat degradation or excessive predation at breeding colonies.
Colony proportions
The ratio of male to female common eiders varies dramatically between breeding and wintering areas. Sex ratios include:
– Breeding colonies – Up to 1 male per 10 females
– Wintering flocks – Closer to 1:1 sex ratio
– Males depart breeding grounds when females begin incubating
– Immature males remain at lower latitudes throughout the year
The skewed sex ratio on nesting areas helps reduce aggressive interactions and forced copulation attempts by groups of males. Mass departures of males help cue females to initiate synchronized nesting.
Brood-rearing habitats
After hatching, common eider females lead their ducklings from nest sites to brood-rearing areas. Typical rearing habitats include:
– Intertidal shoals and flats – Provide invertebrate foods for ducklings
– Estuaries and bays – Offer protected, food-rich sites
– Large lakes – Offer alternate brood habitat inland
– Open coasts – Used where safer sites unavailable
Feeding areas are chosen to provide shelter from weather and concealment from aerial predators while allowing duckling foraging. Females may move broods over several kilometers to optimal rearing areas.
Duckling mortality
Despite attentive parenting by females, common eider ducklings suffer high mortality from:
– Exposure – Small ducklings are sensitive to cold and wet conditions
– Starvation – High calorie foraging needs of ducklings
– Predators – Gulls, eagles, foxes, bears, otters, and more
– Accidents – Ducklings may drown in rock crevices in storms
Mortality averages 50-60% but can reach 90% in harsh weather or with dense predator populations. Females must balance brooding with foraging to maintain their body condition during brood rearing.
Survival and longevity
– Annual survival rates of adult common eiders average 75-85% for females and 60-75% for males.
– Maximum lifespan recorded in wild is 27 years.
– Oldest recorded captive individual was 28 years and 7 months old.
– Experienced breeding females demonstrate higher nest success.
Hunting pressure and marine oil spills take a toll on survival rates in some areas. The long lifespan helps offset early nest failures and supports lifetime reproductive success.
Population threats
Common eider populations face a number of conservation threats:
– Oil spills – Oiling causes mass mortality, nest failure, and habitat loss
– Hunting – Harvests must be monitored and limited
– Predators – Excess foxes, gulls, and corvids on islands reduce nest success
– Human disturbance – Nesting colonies are highly sensitive to disruption
– Climate change – May alter Arctic habitats and food webs
– Toxics – Marine pollutants accumulate in eiders and may affect reproduction
Sustainable harvesting and protection of key nesting colonies are required for continued survival of common eiders. Climate shifts are a concern for the future.
Conclusion
In summary, common eiders are ground-nesting sea ducks that breed along northern coasts and islands. Females select concealed sites near feeding areas and build well-insulated nests lined with down feathers. Despite camouflage, high nesting densities, and aggression by hens, nests often suffer heavy losses to predation. Female nest site fidelity concentrates populations, facilitating monitoring and conservation. A variety of threats must be managed to sustain global eider numbers.
