Factors causing variation in flock size: Decision making to join a foraging flock

In order to clarify the temporal variation of flock size, we examined factors that cause the temporal variation of foraging flock size in a wintering population of the Yellow‐throated bunting ( Emberiza elegans ). We conducted field observations to examine whether the flock size varies greatly and whether non‐random associations between individuals exist. We confirmed that flock size varied greatly and some individuals showed non‐random associations with other flock members. Then, we carried out computer simulations that assume stochastic risky situation, the condition of hunger level and non‐random associations between individuals. In the simulation, the condition of hunger level caused the variation of flock size. However, the distribution of flock size differed from observed ones. When the condition of non‐random association was added to the simulation, the variation of flock size became large and the distribution of flock size was similar to that of the observed one. In wintering flocks of Yellow‐throated bunting, each individual attempts to forage in an optimal flock size. However, if they are extremely starved, it is suggested that they adopt a conditional strategy to join a flock independently of the flock size to acquire the energy necessary for survival. Simultaneously, they may decide to forage under the effect of non‐random association between individuals.