Vigilance is a behavior in birds that is used to detect predators and monitor rivals, and it can be affected by several environmental and social factors, including group size. Here, Black-necked Cranes (Grus nigricollis) were observed in winter in the Yarlung Zangbo Nature Reserve, Tibet, China, to examine the effect of group size on vigilance behavior at both the individual and group levels. At the individual level, individual Black-necked Cranes in large social groups spent less time in vigilant behavior than when in small family groups. At the social group level, the proportion of vigilant individuals decreased, while the proportion of intervals that at least one individual was vigilant increased, with increasing group size. There was a significant group size effect on vigilance behavior in wintering Black-necked Cranes at both of these levels.