Animals gain a variety of information by forming groups. Furthermore, it is known that the larger the group, the faster and more accurately they can respond to information. As an information source, some animals recognize alarm calls given by other species when they detect a predator (eavesdropping on alarm calls). However, few studies have examined whether the alarm calls of other species are more accurately discriminated as the flock size increases. In this study, we played back the alarm and control calls of Japanese tits to flocks of sparrows of various sizes and recorded whether the flocks exhibited fleeing behavior. Our results showed that the larger the flock size, the more accurate the discrimination between the alarm and control calls became. Thus, a new perspective on the benefit of flocking was suggested: the larger the flock, the more accurate the discrimination of information obtained by eavesdropping.