In the east Atlantic during the latter part of the 20th century, small colonies of Northern Gannets (Morus bassanus) grew at a faster rate than big ones. In this species, colony growth occurs largely at the perimeter. Hence, the relative growth rate of a colony should be inversely proportional to the square root of its size. Data from 34 colonies confirmed this expectation. Modelling based on observed population parameters suggested that small colonies could sustain their faster growth rate only by recruiting birds from other colonies. Conversely, while large colonies grew more slowly, they reared more young than they could recruit. This implied net movement of young birds from big colonies to small ones, and showed how a population of gannets could sustain fast growth by founding new colonies.