Group size effect in nutmeg mannikin: between-individuals behavioral differences but same plasticity

When group size increases, animals from a wide range of taxa reduce vigilance and increase feeding rate, the so-called group size effect. This effect requires that group members display plastic behavioral responses both in terms of vigilance and foraging to obtain the security benefit from grouping and/or to cope with feeding competition. Most studies on group size effects have reported mean group changes in behavior. However, individuals' adjustment of behavior and thus their individual contribution to the overall group-level plasticity remain unexplored. Using wild-caught nutmeg mannikins (Lonchura punctulata), small estrildid finches known to exhibit the group size effect, we investigated individual differences in baseline levels and in plasticity of vigilance and feeding behavior. We experimentally manipulated the number of companions foraging with focal birds and noted how they individually adjusted their vigilance and foraging behavior when group size varied. We found that individuals differed consistently in their vigilance level and foraging rate but not in their ability to adjust to experimental variation in group size. Effect sizes for individual consistency in behavior were as large as those for group-level plasticity. Our results reveal high, albeit not maximal, levels of plasticity in both vigilance and feeding behavior for all individuals in this social foraging context.