你的句子有误。这是2017年12月英语六级考试试卷，在网上查看了好几套，发现有两种表达，但都与你的句子不符。

On the western side of the lake are the well-known“Bird Islands", which attract birdwatchers from across the globe.

On the western side of the lake is the well-known"Bird Island", which attracts birdwatchers from across the globe.

▲这是一个倒装句，On the western side of the lake不是主语，而是表语。两句的主语分别是the well-known "Bird Islands" 和 the well-known"Bird Island"。后面的定语从句的谓语动词也都根据被修饰的先行词的单复数分别用了attract 和 attracts。

1 介词短语不可以作主语。这是倒装句。

2 which attract 用的是复数谓语，由此可知which的先行词一定是复数名词。所以bird island必定是拼写错误，应改为复数。所以主句谓语用are。

On the western side of the lake做主语谓语动词用单数还是复数？

湖的西侧是著名的鸟岛，吸引着来自世界各地的观鸟者。 On the western side of the lake are well-known bird island,which attract birdwatchers from across the globe.为什么用are而不是is.

    【答】 On the western side of the lake 是介词短语作表语，放在句首引起全部倒装，are 是系动词，well-known bird island 是主语, 其中的  island 应该改为复数  islands；which是关系代词引导定语从句，并且在从句中作主语， attract 是定语从句的谓语动词， birdwatchers 是 attract 的宾语； from across the globe 是介词短语作  birdwatchers 的后置定语。

       如果 island用单数，句子应该是：

       On the western side of the lake is the well-known bird island,which attracts birdwatchers from across the globe.