Defence strategies of established, familiar rock pipit neighbours were studied in a population breeding at high density. Theoretical arguments suggest that an armistice and some form of cooperative defence should occur between familiar, territorial neighbours when an intruder tries to settle in an occupied area and owners risk losing part of their territories and renegotiating boundaries. Boundary displays were at a low and steady level after neighbours had been claiming their territories for 4 days and owners were then considered familiar and established. Familiar neighbours entered into an armistice when confronted with an intruder in border zones and also in the adjacent area, that is, inside the familiar neighbour's territory boundary. Neighbours were also involved in coordinated evictions of intruders in half of the encounters between two neighbours and an intruder within established territories. Hence, observational data on fighting behaviour of rock pipit neighbours show the existence of armistice and support the hypothesis of cooperative defence among established, territorial neighbours.1997The Association for the Study of Animal Behaviour