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We use previously published and new data from a long-term study of the red-billed gull (Larus novaehollandiae scopulinus) population at the Kaikoura Peninsula to review causes and consequences of population change. Currently the largest colony in New Zealand, the Kaikoura population, has undergone major changes over the past 52 years. In 1994 the population began to decline, and between 1983 and 2003 it dropped by 51%. Fledging success varied markedly between seasons. The major cause of reproductive failure was predation by introduced mammals, especially ferrets (Mustela furo) and feral cats (Felis catus). These predators became a problem when rabbits (Oryctolagus cuniculus) and hares (Lepus europaeus) increased in number, enhancing the predators’ survival over-winter. Periodic failure in the availability of euphausiids, the main food of red-billed gulls, swamping of nests during high seas, and chilling of nestlings during long periods of bad weather also have been set backs to the population. However, in the absence of predation, these factors produced only a temporary impact to the population of long-lived gulls. The decline in both body size and survivorship of adults over the last 5 decades indicates that there are other factors at work, and indicate that the population has experienced some form of unfavorable environmental condition or stress over a long period.