Patterns of flock formations in a wild chestnut thrush population before winter migration

Abstract For some bird species, flock personality composition can reflect a broader spectrum of personality traits of the individuals within those flocks. The personality composition of these flocks may have important consequences for individual survival for coping with environmental change. Few studies have examined flock personality composition for bird species that form monotypic groups prior to significant ecological and seasonal environmental changes. Here, using a wild chestnut thrush (Turdus rubricanus) population, we examined whether seasonal flock composition was associated with the personality traits of flock members prior to their migration season. We measured the exploratory behavior of 208 individuals from the breeding season and non-breeding season (prior to winter migration) in 2019 and the subsequent breeding season in 2020, with 40 of these individuals categorized into seven flocks prior to their 2019 winter migration. We found that flock size of chestnut thrush was negatively associated with Julian date but not with exploratory behavior (represented as flock exploratory scores generated from linear mixed models) prior to winter migration. Over time flock exploratory personality composition changed, with just three individuals changing flock membership. Exploratory score was significantly different among flocks, regardless of changes in flock membership. Furthermore, groups’ exploratory score were repeatable within flocks only after the three individuals changed flock membership, but not evident prior to these changes in the initial flocks. Our results provide some evidence that personality composition is driven by changes in flock membership caused by individual’s flock changing behavior.