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。