Seabird colonies of the Oslofjord: which skerries, which birds, when to stay away

The inner Oslofjord has 14 active seabird reserves (sjøfuglreservater), with another two dozen in the outer fjord, all under Statsforvalteren’s management. Landing is banned 15 April–15 July; a 50-metre marine buffer applies year-round to most reserves. BirdLife Oslo og Akershus has counted them annually since 1971 — one of Norway’s longest-running ornithological monitoring programmes. The biggest active colonies are on Rambergøya, Heggholmen, and the Småskjær off Nesoddtangen.

The rules, on one page

• Access ban: 15 April – 15 July. No landing. No anchoring inside the marine buffer. No entering a reserve by kayak or paddleboard.
• 50-metre marine buffer. Stay at least that far from the shoreline of a reserve in boat. Some areas use an extended 1 April – 1 August period.
• Outside the ban period landing is allowed at most sites but dogs on leash, no camping on vegetation, no fires.
• Enforcement by Statens Naturoppsyn and police. Violation is a criminal offence.

If you are not sure whether a skerry is protected, assume it is and stay off. The reserves are mapped on Statsforvalteren’s portal. If you see a small sign with a bird silhouette, that is your cue.

The headline collapse: hettemåke

The black-headed gull (hettemåke, Chroicocephalus ridibundus) used to be the defining sound of Oslo’s summer skerries. In 1987 Statsforvalteren counted 14,000 breeding pairs across the inner fjord reserves. In 2015, the same count reached 1,417 pairs. A 90 percent decline in thirty years. Closer to home: one generation.

Why? Several things together. Closed landfills removed a summer food source. Warmer winters and changes in small-fish availability shifted the marine diet. Predation pressure from larger gulls (more resilient, more opportunistic) increased. And the long-term decline is tracking all across northern Europe, not just Oslo.

Kavringen, a 16-hectare reserve off Hovedøya, was once the loudest hettemåke colony in the inner fjord. It is effectively silent now, except for the scream of gråmåke — the herring gulls that have taken the space.

Who’s on which skerry

Kavringen. Herring gull (gråmåke), common gull (fiskemåke), cormorant (skarv), eider, long-tailed duck. The old hettemåke rookery, now gull-dominated.

Rambergøya. A low island connected to Hovedøya and Gressholmen by stone causeways. Lime-rich geology and shallow water attract waders and ducks. Roughly 200 bird species have been recorded across the Gressholmen complex historically — one of the best all-round birding spots in the inner fjord.

Bleikøya and Bleikøykalven. On the northeast skerry known as Bleikøykalven, the defined breeding area: black-headed gull, lesser black-backed gull (sildemåke), eider.

Kaffeskjær, Nakkeskjær, Galteskjær (between Nakholmen and Bygdøy). Terns, guillemots, cormorants. Kaffeskjær historically held up to 558 nesting pairs in a single census. Counts from the 1990s.

Husbergøya and Nordre Skjærholmen (Bunnefjorden). Mixed waterfowl colonies — 220+ pairs on Husbergøya, up to 516 pairs of various duck species on Nordre Skjærholmen. Four different goose species documented.

Knerten (one of the Steilene group). Seabird reserve since 1978. Strict access ban 15 April – 15 July.

The numbers that matter

From the BirdLife Oslo og Akershus 2015 seabird count across the inner Oslofjord:

The eider is the good news. Outer-fjord eider populations have been struggling (see the 2020 winter mortality event), but inner-fjord breeding numbers have trended up — probably tied to water-quality improvements after the harbour cleanup.

Outer fjord: a different scale

Leave the inner fjord and the bird life opens up. Ytre Hvaler Nasjonalpark, which became Norway’s first national park with a marine component in 2009, protects a skerry system that holds breeding populations of great cormorant (storskarv), lesser black-backed and herring gulls, common tern, arctic tern, eider, and the occasional European shag (toppskarv). The outer skerries Torbjørnskjær, Heia, Akerøya, Vesleøya are the key reserves.

Færder Nasjonalpark (2013) protects the Bolærne islands, Færder itself, and smaller skerries across to Tjøme. It has recorded 272 bird species across all seasons. It is also the reliable place for white-tailed eagle sightings — the eagles hunt the eider and cormorant colonies.

There are no puffins breeding in the Oslofjord. They nest from Vestlandet northward — Runde, Lovund, Røst. Occasional winter vagrants reach the outer fjord from Skagerrak, but no colonies here. If someone tells you they saw a puffin at Hvaler last June, gently check what they actually saw.

Reading the skerries

A working rule for the fjord: every white-streaked rock you pass is a seabird ledger. Gulls streak the top. Cormorants stain black-green. Eider leave the rocks mostly clean — they nest on the ground in vegetation, not the exposed rock. The density of the white tells you the size of the colony.

If the birds panic off the water as you approach — a sudden dense takeoff, alarm calls, circling — you are too close. Back off to a hundred metres and they will settle. Photographs are not worth the stress cost to the colony.

If you want to see them properly

Binoculars are essential. The distance rules are real and you need optics to make the most of them. 8×42 is the standard birding configuration; borrow a pair if you don’t have one.

For the inner-fjord colonies a half-day private cruise reaches all the Gressholmen-complex reserves and Kavringen without needing a drive. For Færder and Hvaler the full-day tour is long enough to get you to the outer archipelago and back, which is where the big numbers and the eagle encounters are. Either way, the captain keeps you outside the 50-metre buffer. We have been working these skerries for six seasons and we’d rather a quiet distance and clear optics than a close pass and an anxious colony.

If you are kayaking in season, the same rules apply: stay 50 metres off every reserve between 15 April and 15 July. The Mad Goats self-service rental assumes you know this — it’s part of the sjøvett the company counts on.

The terns at Knerten are the loudest thing in the inner fjord between May and August. We hold fifty metres off the island the whole season — flushing a colony off the rock costs the chicks the calories they need to survive their first week.