This study made at Dumfries, SW Scotland, has revealed new information on the ecology and social structure of an urban colony of Lesser Black-backed and Herring Gulls Larus fuscus and L. argentatus. There has been a decline in the numbers of gulls nesting in Dumfries, contrary to a national trend of increasing numbers. These gulls nested socially and almost invariably had another pair within 100 m. Recruitment was almost totally confined to existing groups and new breeding areas were initiated by forming roosting groups on suitable sites. High-density groups were formed on flat roofs, usually on industrial and commercial properties, while low densities pairs occurred in residential areas and these pairs usually nested on chimneystacks. From 2000 to 2007, the public subjected nesting gulls to appreciable and increasing non-lethal disruption, mainly by nest and egg removal. This halved the overall breeding success and resulted in increased movement of nesting sites between years. However at sites not subjected to management, breeding success was high and movement was significantly lower. On low-density sites, nest-removal and proofing against nesting gulls mainly resulted in pairs moving a short distance, and did not reduce the numbers nesting in the immediate area. In contrast, synchronous removal of nests and eggs (or the prevention of breeding by netting or demolition) at high-density nesting sites caused desertion of the site. In the next breeding season, most of these displaced gulls failed to nest nearby, resulting in an appreciable reduction in density of gulls nesting in the immediate area. Further, the gulls did not move as a group elsewhere within the town. It is believed than many either failed to breed in the year after disruption or permanently emigrated from the town, an interpretation that is supported by two other studies. Surprisingly, few of the gulls that had their eggs and nest removed re-laid in that breeding season. These findings should be used to form the basis of non-lethal management of urban nesting gulls, which should be planned on an experimental basis to confirm and quantify the results from Dumfries.