Svaleø lies in the middle of the small inlet called Bløden, which has been closed to all traffic sinke the Risø Atomic Energy Research Establishment was founded in the fifties. It is one of the smallest islands in the fjord and rises only one metre above the normal water level.

The composition of the island’s breeding bird population has varied a good deal through time. In the sixties it was dominated by a dense Black-headed Gull colony and later a smaller Swan- and Herring Gull colony took over. In recent years a Black-headed Gull colony of about 800 pairs has grown up again. Avocet and Common Tern are also well-represented.
In autumn large flocks of Golden Plover and Lapwing often roost on Svaleøen. The many Greylag Geese which roost in Bløden in early autumn also regularly stand on the island.

At extremely low water Svaleø is almost connected to Risø, which enabled a fox to cross over during the breeding season one year. It took the swans’ eggs and carried them back to its young in their lair, where many of the eggs, which had been marked with a date, were found.