Social facilitation of reproductive behaviour has been studied extensively in gulls and terns, but social facilitation of preening has been reported only anecdotally, and has not been previously quantified. We studied a common tern, Sterna hirundo, colony during the summers of 1996 and 1997 to test for socially facilitated preening. Scan sampling provided evidence of spatial and temporal synchrony of preening behaviour. Preening occurred more often than expected in groups of three or more neighbours. Breeding pairs also preened simultaneously more often than expected. In loafing (resting) areas, the proportion of preeners present increased with tern density. Behavioural observations suggest that preening spread from neighbour to neighbour. The observed clumping in preening behaviour could not be explained by differences in date, time of day or weather. Social facilitation of preening and other maintenance behaviour may be an important aspect of group living that is often overlooked. Copyright 1998 The Association for the Study of Animal Behaviour.