Seabirds spend most of the year dispersed across open ocean and then gather to breed in extraordinary densities on a small number of sites. The concentration is driven by where safe nesting is possible and by what crowding provides once birds are there.
Predator exclusion sets the location
Eggs and chicks on the ground are highly vulnerable, and seabirds cannot defend a nest against a mammal the way a larger animal might.
Sheer cliffs and offshore islands are inaccessible to foxes, rats and other ground predators, which removes the main source of loss during the weeks a chick is immobile.
The narrowness of suitable sites explains the density. There is little safe ledge available relative to the number of birds needing it, so occupancy is packed rather than spread.
Distance to food constrains the rest
A breeding adult must return regularly with food, so a colony can only exist within commuting range of reliable feeding grounds.
Productive waters occur where currents force nutrients toward the surface, and these areas are patchy, which further limits where colonies can form.
Because many thousands of birds forage from one point, the sea immediately around a large colony is depleted, and adults commonly fly well beyond it before feeding.
What crowding provides
Dense nesting allows collective defence, and gulls, terns and skuas will mob an approaching aerial predator in numbers that a single pair could not manage.
Any individual nest is also statistically safer within a large group, since a predator can only take so many and the odds for each pair improve as the colony grows.
Colonies additionally function as information centres, with birds following successful neighbours out to sea and locating prey patches they had not found independently.
The costs of living together
Disease and parasites spread readily at high density, and outbreaks in seabird colonies can affect a large share of a regional population within a single season.
Competition for the best ledges is intense, and neighbouring adults will attack chicks that wander, so much of a colony's noise is territorial rather than social.
Waste accumulates and fouls sites, though it also fertilises surrounding waters and island vegetation, which is a substantial nutrient transfer from sea to land.
Why colonies are fragile
Concentration means a single event at one site can affect an entire regional population, whether that is an oil spill, a storm surge or the arrival of rats.
Site fidelity is strong, with birds returning to the ledge where they were raised, so a colony that fails does not simply relocate the following year.
Recolonisation is correspondingly slow, and restoration projects use decoys and recorded calls precisely because arriving birds look for evidence that others are already breeding there.