SummaryThe breeding biology of Lesser Black‐backed Gulls was studied on Skokholm Island, Pembrokeshire, where the number of breeding pairs has been increasing at about 20% per annum since 1963. Laying was found to be synchronous within small groups. Clutch size and breeding success showed seasonal declines over the spread of breeding. The loss mainly of eggs, but to a lesser extent of chicks also, caused this overall decline in success. Hide observations indicated that the bulk of these losses arose through predation by nesting adult Lesser Black‐backed Gulls on their nearest neighbours. Not infrequently the protagonist had lost its own clutch shortly before turning predator, and such a chain‐sequence should lead to a steady build‐up of aggressive failed breeders and so account for the observed seasonal increase in egg loss. Attentiveness to the clutch decreased with season but this was unlikely to have been important in precipitating the predation. Intermediate plant cover was associated with highest nest density and also with highest chick survival. In addition, nest density was directly correlated with chick survival. Whether plant cover and nest density separately affect chick production remains unresolved. Nevertheless, the increasing nest densities in this colony caused by the growth of the gull population are thought to be responsible for the widespread intraspecific predation, the intensity of which is probably a new feature of the gulls' breeding behaviour. The implications of this ‘internal’ predation for laying synchrony and aggregated nesting are discussed; these two factors of the breeding pattern probably evolved largely to combat ‘external’ predators. Not only are they no protection against inter‐neighbour predation but appear to facilitate it. It remains to be seen whether this kind of predation will significantly affect breeding patterns with further increase