Quails typically have short rounded wings that enable them to spring into f

题目

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!

答案