We do a bird count once a year.(对画线部分提问)划线部分是once
问题描述：
We do a bird count once a year.(对画线部分提问)
划线部分是once
答
用how often，once表次数
所以提问应当问频率
答
how often do we do a bird count?
我们多久数一次鸟
答
How often do you do a bird count a year?问频率
答
How many times do you do a bird count a year?