Where does a flock end from an information perspective? A comparative experiment with live and robotic birds

Predator detection is improved when individuals join groups. Theory assumes that the transfer of social information about predators among individuals is immediate and accurate. However, animals in groups space themselves at different distances. Little is known about the shape of the social information transfer function over distance, which can affect group cohesion and ultimately the costs and benefits of group living. Our goal was to study the flow of social information in 3 bird species with different visual acuity (European starling, brown-headed cowbird, and house finch). We used robotic birds to manipulate the availability of social information. In a previous study, we demonstrated that birds react to robotic birds in the same way as they do to live birds. We measured the probability of 3 linearly placed live birds reacting to the flushing behavior of 2 robotic birds of the same species. Our study species were tested independently. We found a nonlinear decrease in social information flow with increasing distance between the robots and live birds; however, this decrease was more pronounced in species with lower visual acuity. Additionally, social information apparently degraded when flowing between closely spaced individuals, which could lead to false alarms. Our findings suggest that the benefits of social information flow are restricted to small neighbor distances and that larger species, with higher visual acuity, may have a greater spatial domain of collective detection. This mechanism may explain the spatial limits of flocks based on the transfer of social information.