Question

Which figure of speech does Thoreau use to show how loud the bird was in the sentence: "The wingspanerels crow on the trees, clear and piercing for miles over the resounding earth! It would put nations on the alert."?

Answer 1

Well, first you need to look at what he says to describe the sound of the birds:
-clear and piercing for miles over the resounding earth
-it would put nations on the alert

Which one of these is a fact and which one is an imagined idea?
It is a fact that they were clear and piercing and it was a fact that he could hear them from far away. (though probably exaggerated)
The figure of speech is "it would put nations on the alert" because the birds would not literally put nations on the alert.