The Channel Island of Jersey is 116 km2 but has only 540 ha of woodland distributed in 237 wooded fragments with mean size of 2.5 ha. Despite this, the island supports 300 to 600 red squirrels Sciurus vulgaris, a species that is under threat in mainland Britain from habitat fragmentation, disease and the spread of the alien grey squirrel S. carolinensis. Grey squirrels are not present on Jersey. Between 1994 and 1997, using trapping and tracking techniques, we studied how red squirrels survive on Jersey by focusing on three local populations in sites B, C and S with contrasting food availability. The primary squirrel foods at site B were autumn tree seeds, at site C autumn tree seeds and summer conifer cones, and at site S autumn tree seeds and ad libitum, year-round supplementary food provided by local people. The densities of squirrels in site C (mean over 3 years 0.97 ha−1) and site S (1.21 ha−1) were more than one and a half times that of site B (0.68 ha−1), which relied on autumn seeds. Body mass and condition, and early breeding (spring litters) were not affected by food availability although conception occurred as early as November, earlier than in mainland Britain. Less than 50% of females produced summer litters, and 88% of non-breeders did not receive supplementary food. Turnover rates were lowest and most stable in site B. Annual residency was not influenced by the size of the natural tree seed crop. Cat and road kills caused two out of three reported deaths. This island population seems to remain viable as a result of an extended breeding season and increased population densities in sites with summer or supplementary food. The results emphasize the beneficial effects of extra food provided by local people. This provides a buffering effect against variations in natural food availability, and seems to be important for their long-term survival.