On the days when she wore a skirt the men on the building site would whistle at her.

whistle 是动词，意思是"吹口哨"，指用嘴唇和舌头发出高音调的声音。在这个句子中，whistle at sb 表示"对某人吹口哨"，通常带有调戏或骚扰的意味。