Black-headed gulls synchronize vigilance with their nearest neighbor irrespective of the neighbor's relative position

Behavioral synchronization refers to actions conducted simultaneously by two or more individuals at the same place. Through behavioral synchronization, animals may increase social cohesion of their groups or achieve antipredator and foraging benefits. We examined whether black-headed gulls (Chroicocephalus ridibundus) synchronized vigilance with their nearest neighbors and whether synchronization was affected by the neighbor's relative position. We predicted that (1) the synchronization of vigilance is less likely when neighbors are positioned behind focal individuals, i.e., within the blind sector of their visual field, and that (2) birds may show preference for copying vigilance when neighbors are situated at one side due to known laterality in avian eye use for antipredator vigilance. Data were obtained for 46 pairs of individuals—focal gulls and their nearest neighbors—through the observation of video-recorded material. Gulls significantly synchronized vigilance with their nearest neighbors irrespective of the neighbor's relative spatial position—the odds that the focal individual would scan its environment were increased almost three times if the nearest neighbor was also vigilant. Contrary to our predictions, however, we did not find evidence that synchronization of vigilance in gulls was dependent on their nearest neighbor's relative position, which could be due to several reasons: (i) gulls may pay attention not only to their nearest neighbors but also to more distant ones and/or (ii) gulls may have reacted to some cues other than visual, such as alarm sounds. Although the mechanism of synchronization in gulls remains unknown, by being vigilant at the same time as other group members, the gulls may achieve additional antipredator benefits.