英语翻译Molt of the flight feathers is the most highly organized

英语翻译
Molt of the flight feathers is the most highly organized part of the process.Some species,for example,begin by dropping the outermost primary feathers on each side(to retain balance in the air) and wait until the replacement feathers are about one-third grown before shedding the next outermost.
这篇文章讲的是关于鸟掉毛的.
上面这两句话怎么理解?
尤其是第二话中的三分之一,是指什么的三分之一?长出了掉的羽毛的三分之一,还是总共羽毛的三分之一?说这种鸟为了维持平衡,只掉三分之一的羽毛对不对?