Group living is only predicted to arise when the fitness benefits outweigh the costs of sociality. Group-living species—including cooperatively breeding and family-living birds and mammals—occur most frequently in environments where climatic conditions fluctuate unpredictably from year-to-year. The fitness consequences of group living are thus expected to vary with changing environmental conditions, though few studies have examined this possibility. We examined whether living in large social groups improves adult survivorship in cooperatively breeding superb starlings (Lamprotornis superbus). We also tested the hypothesis that larger groups buffer against harsh conditions by increasing survivorship most under periods of low rainfall. We found that group size was positively correlated with adult survival, but in a sex-specific manner: female survival increased with group size across all environmental conditions, whereas male survival only increased with group size in wet years. Together with previous work in this system, our results suggest that larger groups confer survival benefits by reducing predation, rather than improving access to food or buffering against physiological stress. Although group living does not appear to buffer against harsh conditions in adult starlings living in a fluctuating environment, living in larger groups does confer a survival advantage.