Disease-driven variation in body reserves links social rank, flock size, and overwinter survival in greenfinches

Social living offers both benefits and costs, particularly when infectious disease alters the balance between competition, energy reserves, and survival. In flocking passerines, individuals differ in dominance rank and access to resources, which may shape body condition and overwinter survival in a context-dependent manner. We studied wintering greenfinches (Chloris chloris) to examine how social rank, flock size, and trichomonosis outbreaks interact to influence body reserves and survival until spring. Over multiple winters, including years with and without a trichomonosis epidemic, we measured body mass, wing length–based body mass index (BMI), dominance rank, and overwinter survival of individually marked birds across flocks spanning a wide range of sizes. Body reserves varied with dominance rank in a flock-size–dependent manner: in medium-sized and very large flocks, higher-ranking individuals tended to maintain higher BMI than subordinates, whereas no clear rank-related differences in BMI were detected in small flocks. These rank-dependent patterns were further modified during epidemic winters in a flock-size–specific way, with altered dominance–BMI relationships particularly evident in medium-sized flocks. Overwinter survival increased with BMI during the epidemic, indicating that greater body reserves enhanced the likelihood of surviving to spring under disease pressure, whereas outside epidemic conditions BMI showed a weak negative association with survival. Together, these results demonstrate that disease-driven environmental conditions modify the relationships between social structure, energetic strategies, and survival. Our findings highlight how infection risk can reshape adaptive trade-offs between dominance, body reserves, and fitness outcomes in social birds.