Sarah Guindre-Parker and Dustin R. Rubenstein (June 2020)
Group living does not buffer against harsh conditions in starlings, but living in larger groups does improve survival
“Survival benefits of group living in a fluctuating environment”
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Animals that live in large social groups can face increased competition for food or mates from other group members. Yet, group living persists in many species despite this increased competition—as a result, biologists expect that some advantage must exist to living in large groups at least under some contexts. One possibility for animals living in unpredictable environments is that group living is beneficial in order to overcome harsh environmental conditions where challenges cannot be anticipated.
Guindre-Parker and Rubenstein studied a population of superb starlings in East Africa for over 15 years. These birds experience unpredictable variation in rainfall from year to year, and live in some of the largest social groups known for a cooperatively breeding bird (ranging from 7 to 57 individuals per group). The researchers monitored individual birds over their lifespan using unique combinations of colored leg bands and extensive field observations. The team tested whether survival was higher for individual birds belonging to the largest social group compared to small groups. They also examined whether living in the largest social group improved survival most under periods of harsh, low rainfall, compared to years with abundant rain.
This study found that female superb starlings benefited from living in larger social groups, since female starlings had longer lifespans when they belonged to the largest groups. This result held true regardless of rainfall conditions, however, suggesting that female birds always benefit from living in large groups regardless of harsh conditions. A similar pattern was observed in males, where male starlings had the longest lifespan if they lived in larger social groups. However, living in larger social groups improved male survival most when rainfall was abundant. In other words, group living did not improve male survival under harsh conditions as previously expected, but instead improved male survival most under benign conditions. Overall, the authors conclude that group living provides important survival benefits to superb starlings, but group living does not appear to buffer group members against harsh periods of low rainfall and food availability.
Abstract
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.