Black-necked Cranes (Grus nigricollis) wintering at Dashanbao Nature Reserve in southwestern China were studied to document for the effects of flock size and position on the level of vigilance. The time spent on vigilance, scanning frequency and scanning duration decreased significantly with flock size for individuals positioned both in the center of the flock and at its periphery. However, the decrease in time spent on vigilance and scanning frequency following the increase in the flock size happened more rapidly for individuals in the center of the flock than those at the periphery. The decrease of the three parameters related to flock size significantly correlated with position. The time spent on vigilance and scanning frequency significantly correlated with the flock size and position, but not with scanning duration. Consequently, the number of individuals preferring central positions in the flock (a phenomenon known as “edge effect”) was increased by the increase in flock size for Black-necked Cranes in winter.