Graesholmen is the third island of the Ertholmene archipelago, lying just to the north. No-one is allowed to land — the entire island is a strict nature reserve set aside for breeding seabirds. From the north shore of Christianso, you can look across the narrow channel with binoculars and see the colonies on the cliffs.
Spring and early summer are the best time. The smell will reach you before the sound. Bring binoculars; the regular Christianso birders are happy to point out the rarities. Migration in late autumn brings smaller songbirds through in tens of thousands.