Feral cats were introduced to Ascension Island in the 1800s and extirpated all seabirds with the exception of Sooty Terns to inaccessible islets, stacks and cliffs. Sooty Terns continue to breed in reduced numbers on the south-west plains of the island and are still subject to cat predation, so monitoring of their population trends is important. Measuring the impact of cat predation upon seabird populations depends on having reliable baseline data; censuses of Sooty Terns on Ascension were conducted in 1990, 1996, 1997 and 1998 and involved sampling clutch densities in sub-colonies and then extrapolating to the total colony area. The population varied significantly over the study period, with 176 000 pairs in 1990, 202 000 pairs in 1996, 151 000 pairs in 1997 and 207 000 pairs in 1998. The 22% reduction in 1997 compared with 1996 and 1998 is thought to be due to a large proportion of mature birds deferring breeding because of reduced food availability induced by oceanographic perturbations. Such variability in breeding population size in relation to stochastic events means that censuses need to be undertaken frequently to ensure trends can be detected with confidence.