when they are flying 是不是时间状语从句

只有when they are flying 不是时间状语从句。如果再加一个主句，when they are flying就 是时间状语从句。也可能是宾语从句等。

例句：

The birds are singing when they are flying.

鸟飞翔时，它们唱着歌。

when they are flying 是时间状语从句。

I want to know when they are flying.

when they are flying 不是时间状语从句， 而是宾语从句。