Plizio Visual Lab

Ertholmene

Plizio editorial teamUpdated: Jun 2026Data Sources

wildlife

Weather…

Ertholmene is a small Danish archipelago in the Baltic Sea, located northeast of Bornholm, consisting of the inhabited islands Christiansø and Frederiksø, along with several uninhabited rocky islets. The history of the islands is defined by their military past: in 1684, King Christian V established the first fortified naval base for the Danish Navy in the Baltic Sea here. The fortress architecture, featuring iconic round towers and distinctive yellow buildings, has remained largely unchanged since its construction. Following its decommissioning as a military stronghold in 1855, the islands were placed under historical protection and have since become a significant nature reserve. Ertholmene is particularly famous for its large breeding colonies of common guillemots and razorbills. The entire archipelago is car-free and is administered directly by the Danish Ministry of Defence, granting it a unique administrative status within the kingdom.

Location: Denmark
Top sights: Tat Fyr · Christiansø Museum · Statsfængslet 'Ballonen'
Nearby: Gudhjem (18 km)

Best time to visit & climate

The most pleasant time to visit is Jul–Sep.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Avg °C	3	2	3	5	10	14	17	18	15	11	8	5
Rain mm	60	48	42	32	42	46	69	70	56	70	69	65

📋 Practical info

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Best season

The best time to observe rare seabirds on Ertholmene is from April to July. During these months the birds breed and the colonies are most active.

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Best time of day

Early morning the birds are busiest when they go foraging. Also in the late afternoon many return to their nesting sites.

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Etiquette

Only enter the islands on marked paths to avoid disturbing the breeding colonies. Keep distance from nests and avoid loud noises.

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What to bring

A good pair of binoculars is essential for observing seabirds from a distance. Waterproof clothing and sturdy footwear are recommended for the boat trip and island terrain.

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Getting there

Ertholmene is only accessible by boat from Bornholm, usually from Gudhjem or Allinge. The crossing takes about one hour and is operated by local ferry companies.

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What to expect

The birds can be shy and may keep their distance. Allow plenty of time and find a quiet observation spot.

Geography

Coordinates

55.32000, 15.19000

Facts

The fortress on Christiansø was founded in the year 1684.
The archipelago is situated roughly 18 kilometers northeast of Bornholm.
Currently, fewer than 100 permanent residents live on the islands.
Ertholmene provides the only Danish breeding grounds for razorbills.
The islands have been administered by the Danish Ministry of Defence since 1855.
There are no cars on the islands, and pets such as dogs and cats are prohibited.