Quails typically have short rounded wings that enable them to spring into fquails typically have short rounded wings that enable them to spring into full flight instantly when disturbed in their hiding places.中的“when disturbed”为什么不是"when they are disturbed" 这个用法是什么语法,似乎对理解没问题,但是感觉总缺点什么成分!

Quails typically have short rounded wings that enable them to spring into f
quails typically have short rounded wings that enable them to spring into full flight instantly when disturbed in their hiding places.中的“when disturbed”为什么不是"when they are disturbed" 这个用法是什么语法,似乎对理解没问题,但是感觉总缺点什么成分!