Winter birds on the Oslofjord: eider, goldeneye, and the mystery of 2020

From November to March the Oslofjord hosts tens of thousands of overwintering diving ducks. The four core species are common eider (ærfugl), goldeneye (kvinand), long-tailed duck (havelle) and velvet scoter (sjøorre). The inner fjord rarely freezes solid; the Drøbak narrows and inflow from Akerselva keep the basins open. Mussel beds along every rock wall feed the birds through the cold months. February is peak count.

The lineup, with red-list status

Categories are from the 2021 Norwegian red list, which assesses the Norwegian population. Most of the birds you see on the fjord in January bred somewhere else, so the category is context rather than a headcount of what is in front of you.

The core species

Common eider (ærfugl)

This is the defining winter duck of the Oslofjord, Somateria mollissima. Females are brown and well camouflaged. Males are unmistakable: black-and-white body, pale pink-washed breast, green nape patch, long sloped bill. They dive for blue mussels, sometimes 15 metres down, and crunch them whole. You can hear the crunching from a quiet boat.

Winter concentrations run to thousands in the outer fjord. In February the males begin their courtship displays, the best soundtrack of an Oslofjord winter: a deep rolling ooh-hooo that carries across a half-mile of flat water. It is the one call on this list you will still recognise a year later.

Goldeneye (kvinand)

A small diving duck, Bucephala clangula. Males are black-and-white with a distinctive round white spot on the face below the eye. The species breeds inland in Norwegian lakes and moves to coastal waters for winter, a classic inland-to-coast seasonal shift. They dive for invertebrates and small fish. In January you see clusters of twenty or thirty in the shallower Oslo harbour basin.

Long-tailed duck (havelle)

The most spectacular of the winter ducks, Clangula hyemalis. Males have extraordinarily long tail feathers that trail behind them in flight and on the water. BirdLife Norge made it Bird of the Year in 2015. They breed in the Arctic and winter along the Norwegian coast.

Conservation status is worse than for the other ducks here. Surveys of the Baltic wintering population found a fall from about 4.27 million individuals in 1992–93 to about 1.49 million in 2007–09, a decline of roughly 65 percent (Skov et al. 2011). That survey is what put the species on the IUCN Vulnerable list. The Norwegian breeding population is small, 13,000 to 27,000 individuals and listed near threatened, but the winter birds passing through the Oslofjord come from across the whole Arctic.

Velvet scoter (sjøorre) and common scoter (svartand)

These are large all-black diving ducks. Sjøorre (Melanitta fusca) is Norway's second-largest diving duck after eider. Males are black with a distinctive white wing patch and white eye ring. They can hold a dive over a minute chasing mussels 30 metres down. You watch a flock on the surface, they all disappear at once, and they reappear a hundred metres away. Svartand (Melanitta nigra) is the smaller all-black relative, and less common in the Oslofjord than sjøorre. Both sit in the vulnerable category on the Norwegian red list, sjøorre on the size of its breeding population and svartand on decline.

Goosander and red-breasted merganser (laksand and siland)

These are the sawbills, large diving ducks with thin hooked bills serrated for catching slippery fish. Laksand (Mergus merganser) breeds inland on rivers and moves to the coast in winter. Siland (Mergus serrator) breeds and winters along the fjord, one of only two duck species in Norway that commonly breeds on the coast itself (eider is the other).

Guillemots and auks (lomvi, alke, teist)

In certain winters, guillemots invade the inner Oslofjord in numbers. BirdLife Oslo og Akershus runs the counts that pick these years up. Nobody can predict the invasions, which are driven by food availability in the northern North Sea. If it happens, it becomes the birding story of the season. It is also the scarcest bird on this page: the Norwegian breeding population of lomvi is down 85 to 90 percent from the 1960s and the species is critically endangered.

Where to find them

• Bygdøysund. Eider, goldeneye, merganser. Accessible from the Huk peninsula by foot or bus.
• Frognerkilen. Sheltered inner bay. Ducks plus mallards, gulls. The easiest city-edge winter birding, walkable from central Oslo.
• Drøbaksundet. The narrows. Eider, long-tailed duck, scoter. Auk invasion site when they arrive. View from Drøbak harbour or the ferry crossing.
• Hvaler archipelago. The main eider wintering ground, thousands of birds. Also scoter and long-tailed duck concentrations.
• Kurefjorden (Østfold). Major wader and waterfowl migration stopover. Best in October and April for passage migrants.
• Nordre Skjærholmen (Bunnefjorden). A quiet corner of the inner fjord with a small mixed waterfowl colony in summer and gulls, eider and geese around it the rest of the year.

Why the fjord stays open

Four things keep it open at once. The water is salty, brackish in the inner basin and fully marine further out, and salt water freezes at roughly -1.8°C instead of 0°C. The deep basins (Vestfjorden, 200+ metres) hold enough thermal mass to resist freezing. Akerselva, Drammensvassdraget and other rivers discharge warmer freshwater into the inner basins. And the narrows at Drøbak funnel the tidal current strongly enough that freezing is physically prevented.

Bunnefjorden is the exception. In harder winters the innermost end, east of Nesoddtangen, can freeze. In 2024 VG reported a small group of porpoises trapped under ice in Bunnefjorden that winter, unusual enough to make the news.

The spring 2020 eider die-off

In March 2020 dead and dying eider began turning up along the shores of the outer Oslofjord and eastern Agder. Statens Naturoppsyn collected more than a hundred, and NINA took 104 of them to Trondheim for autopsy. The birds were emaciated: mean condition index 0.7 across 87 individuals, which is the category NINA describes as fatally starved. Thiamine (vitamin B1) levels measured in brain and liver tissue came back alarmingly low, a signal that points at the prey base rather than at the birds.

The leading hypothesis involves wrasse. With cod largely gone from the fjord, wrasse and crab populations have expanded, and wrasse eat the blue mussels that eider live on. Researchers put the theory to Dagsavisen in April 2020 and NINA has monitored the situation since. There is no closed answer yet.

The timing invited a different theory. On 23 February 2020 the M/V Trans Carrier lost 13.2 tonnes of plastic pellets into the Skagerrak, and much of it drifted onto the Østfold coast and into the outer fjord. NINA examined 50 of the dead eider specifically for pellet ingestion. A few pellets turned up in two birds, and the conclusion from Kystverket was that the deaths were caused by starvation and could not be linked to the plastic.

Migration timing

• October–November: arrivals from the north. First eider, goldeneye, merganser. Numbers build through November.
• December–February: peak. Thousands of eider in the outer fjord, hundreds of other species across inner bays.
• Late February: eider courtship calls begin. This is the clearest signal winter is ending.
• March: departures start. Inland species move back inland. Long-tailed duck begins heading to the Arctic.
• April: most winter birds gone. Oystercatcher (tjeld) returns, the traditional signal of spring on the coast.

If you want to see them

Bring binoculars, warm clothes and a thermos. Winter birding in the Oslofjord is mostly a land-based activity, because most tour boats stop running for the winter, and ours is on land from November to March. Almost every stretch of fjord waterfront is walkable, and most of the best spots are reachable by Ruter bus or a short drive.

BirdLife Oslo og Akershus runs organised winter counts and posts reports at oa.birdlife.no. National seabird data is aggregated through SEAPOP. For species status and red-list updates, Artsdatabanken.

When our season opens again on 1 April, some winter birds are still around and the first migrants are back. A late-April private cruise is a good shoulder-season window: the last of the eider courtship displays, the first terns arriving, and the fjord emptier of boats than it will be all summer. For the wider winter picture we have a broader piece on Oslofjord winter: floating saunas, ice swimming, the fjord under low Nordic light.

Walk out to Huk in February and you can hear the first deep rolling ooh-hooo from a drake a hundred metres off the beach. Norwegian coastal culture has been organised around that bird for a long time. On the Vega archipelago in Nordland, islanders sheltered nesting eider and harvested the down for generations, and that tradition was the main reason Vegaøyan entered UNESCO's World Heritage list in 2004.

Red-list categories are from the 2021 assessment and are revised roughly every seven years. Population figures move with each survey. Last reviewed: July 2026.

January is when the boats are out of the water and the islands are deserted, but the fjord itself is full. Goldeneye in Bunnefjorden, eider rafts off Steilene, the occasional long-tailed duck near Hovedøya.