The effect of distance between members of pairs of the Ruddy Shelduck (Tadorna ferruginea) during vigilance behavior was studied during winter at Lijiang Lashihai Lake Reserve, southwest China. The distance between the paired birds while feeding was grouped into three categories: <1 m, 1-3 m and >3 m. Nearly 90% of the distances recorded between paired males and females were within three meters. The degree of vigilance increased with distance apart for females, but not for males. The scanning rate of males was significantly higher than that of females at <1 m and at 1-3 m respectively, but no significant difference occurred when the distance between them was >3 m. These results are discussed in relation to mate competition and wintering strategy; it is suggested that staying close together is the optimal strategy for members of Ruddy Shelduck pairs.