Why do many pelecaniform birds breed in colonies?

Many pelecaniform birds breed in colonies primarily due to the scarcity of safe nesting sites. Because these birds require locations free from mammalian predators—such as isolated oceanic islands, cliffs, or trees standing in water—they are often forced to crowd into the limited available space that meets their safety requirements.1

In many species, this colonial behavior has become obligatory, meaning individuals or small groups are unable to breed successfully on their own. The density of these colonies is typically regulated by the distance an incubating bird can reach to defend its nest against intruders. While some species may nest in dispersed patterns when space is abundant, the overall tendency toward colonial breeding is a direct response to the restricted availability of secure environments.1