when they are flying 是不是時間狀語從句

只有when they are flying 不是時間狀語從句。如果再加一個主句，when they are flying就 是時間狀語從句。也可能是賓語從句等。

例句：

The birds are singing when they are flying.

鳥飛翔時，它們唱著歌。

when they are flying 是時間狀語從句。

I want to know when they are flying.

when they are flying 不是時間狀語從句， 而是賓語從句。