Long-chain omega-3s (n3-LCPUFAs) are produced primarily in aquatic ecosystems and are essential for the structural integrity of vertebrates’ brains. Carnivorous mammals must consume n3-LCPUFAs throughout their lifespan to maintain optimal cerebral functions; it is unknown whether other taxa are under similar nutritional pressures. This is concerning because many avian predators now rely on anthropogenic foods lacking n3-LCPUFAs. Here, we tested whether a recent or longer-term diet explains the encephalic fatty acid composition of a seabird, the Ring-billed Gull, that now thrives in cities. During the breeding season, rural gulls exploiting marine organisms had significantly higher encephalic levels of n3-LCPUFAs (mean±SD: 32±1%) than city-nesting gulls exploiting garbage (27±1%). Stable isotope analysis of blood and feathers grown during different seasons showed that urban and rural nesters consumed similar fall and winter diets, suggesting that the difference in cerebral n3-LCPUFA of breeding adults was due to concomitant and transient dietary differences. We also experimentally manipulated gulls’ diets throughout incubation with a n3-LCPUFA supplement, a n3-LCPUFA-free caloric equivalent, or nothing, and found evidence that supplemental n3-LCPUFA increased urban nesters’ cerebral levels of n3-LCPUFAs. These complementary analyses provide strong evidence that the brain of a seabird remains plastic during adulthood and responsive to short-term dietary changes.