Social birds copy each other's lateral scans while monitoring group mates with low-acuity vision

Copying others can be used to enhance foraging and mating opportunities, but can be costly due to the need to monitor the actions of others, which can take time away from foraging and antipredator vigilance. However, little is known about the way animals monitor conspecifics. We investigated the mechanism that European starlings, Sturnus vulgaris, use to visually monitor group mates in perching situations through two questions. First, do starlings copy the timing of each other's vigilance? Second, do they use their centres of acute vision to monitor group mates? We studied a component of vigilance that has received relatively little attention, lateral scans, which consist of changes in the orientation of the head (i.e. gaze shifting) while in a head-up position. We found that starlings copied the timing of their neighbour's scans, placing them closer together in time than expected by chance. This could enhance the speed of social information spread within a group compared to random timing of head movements. The strength of this copying effect varied with neighbour distance and the sex of the follower and leader, suggesting that starlings appear to be more motivated to copy some individuals over others. Additionally, instead of monitoring neighbours with their centres of acute vision (high-quality vision), starlings tended to use their retinal periphery (low-acuity vision), potentially reducing the costs of social monitoring. Copying the timing of lateral scans may have advantages for gathering social information (i.e. quick responses to movements of group mates in situations such as murmurations). However, it can also have costs in terms of delaying the detection of personal information by any group member (e.g. predator attack) due to longer gaps without changes in gaze orientation. Therefore, copying the timing of lateral scans may be restricted to specific contexts (e.g. low predation risk).