Migrant robins Erithacus rubecula wintering in the Mediterranean area defend winter territories against conspecifics, black redstarts Phoenicurus ochrurus, and stonechats Saxicola torquata. The characteristics of 14 territories and the factors influencing the owners' attacks against intruders were studied during the winter 1992-93. Territories showed a well-defined core area of dense bushes (mean = 0.037 ha, 74 m of perimeter, < 5 m tall with 1-7 shrubland species) systematically used by the owner as a shelter, roosting and searching point. Fleshy fruits (e.g. Pistacia lentiscus) were also available in 10 of these territories. Territory size, including open area around the core area also defended by the owner, ranged from 0.04 to 0.30 ha (mean = 0.17, n = 10 ringed robins). Of 188 intruders recorded, 79 elicited an attack by the owner (on average 0.9 intruders and 0.3 attacks per 30 min session, n = 192). The attack rate was also higher in early (November - January) than late (February and March) winter. The attack against intruders was consistently more pronounced 1) against conspecifics than against intruders of the other two species, 2) when temperature of previous days had been colder and 3) the number of intruders per session was lower. The results suggest a remarkably small size of Robin territories in the study areas and an intense intra- and inter-specific competition for resources in winter. Although the functional explanation of nonbreeding territories is unclear, territories seem to be a crucial resource that may significantly affect the behaviour, distribution, and presumably, survival of the Robin in winter.