Græsholm, the island to the northwest of Christiansø, is a wildlife refuge and an important breeding ground for guillemots, razorbills and other seabirds.

Together, Christiansø, Frederiksø and Græsholm are known as the Ertholmene Islands, and they serve as spring breeding grounds for up to 2000 eider ducks. The ducks nest near coastal paths and all visitors should take care not to scare mothers away from their nests because predator gulls will quickly swoop and attack the unattended eggs.

The islands also attract a colony of seals.