383. Other animals, it is true, communicate with one another by means of cries: for example, many birds utter warning calls at the approach of danger; monkeys utter different cries, such as expressions of anger, fear and pleasure.
it is true为插入语。
The police communicate with each other by radio.警察通过无线电相互联络。
by means of意为“以…方式，通过…的手段”。如:
The theory is illustrated by means of Power Point.该理论以 PPT的形式进行讲解。
Why do you utter a sign?你为什么要发出叹气声呢?
at the approach of意为“在……将到的时候”。如:
A bird has to give way at the approach of an elephant.大象走过来，小鸟要让路。