With Common Terns for example, nesting islands must be free of land predators most of the time. This is important as terns are a ground-nesting species most often found in an open area colonized only by grasses. An area free of extensive predation is not the only requirement for terns to colonize a site.
Terns fly low over the water and dive for small fishes below the surface of the water.
This provides food for the adults or hungry young. Feeding and breeding areas must therefore have clear water, abundant food resources, and few predators. Cliffs composed of unconsolidated materials are important to birds that nest in burrows, such as bank Swallows.