The function of the buffer zone of nature reserve is seldom evaluated in wildlife conservation. National Yancheng Rare Birds Nature Reserve is the largest coastal nature reserve in China. The environmental capacity of its core zone of 225.96 km2 area is approaching saturation, and more and more birds have begun to utilize the buffer zone. We conducted a line transect survey in December 2010 to estimate bird density and evaluate the importance of the buffer zone. As a result, 34 protected species among the 4975 individuals were sighted, including 127 red-crowned cranes (Grus japonensis) and 868 common cranes (Grus grus). The overall density of protected species, winter migratory species, and resident birds was 1244.60 individuals/km2, 396.88 individuals/km2 and 758.94 individuals/km2 respectively. The woodland, aquaculture, and farmland with >200 individuals/km2 each signified the vital habitat of birds. Comparatively, density in the original habitat (woodland) is relatively higher than that in artificial habitats (aquaculture and farmland), which reached 81.0%, 13.9% higher than aquaculture and farmlands. These observations suggest that the northern buffer zone comprises of vital habitats for bird population. We recommend that priority should be given to the northern buffer zone to preserve the original habitat, whereas conversion of the original habitat to aquaculture and farmland use should be given great concern.