Means (±1 standard deviation) of bill measurements (in millimeter) for mallards by group and sex.

See Table 2 for sample sizes. Different letters indicate significant difference of means within each group and sex.Means (±1 standard deviation) of bill measurements (in millimeter) for mallards by group and sex.样本量详见表2。各组及各性别亚组内的均值若标注不同字母，则表明存在显著差异。按分组与性别划分的绿头鸭喙部测量值（单位：毫米）的均值（±1倍标准差）。