The second largest colony in North-Rhine - Westphalia now breeds at the Birgelfeld site. Thus, the gravel pit contributes to the survival of the species. In connection with other locations, a network of nesting sites was created in the sense of a metapopulation. This minimizes negative influences and stabilizes the population as a whole. The protection of Common Tern is one of the most successful conservation projects in North-Rhine - Westphalia.