The author uses creatures in some instances to make the reader identify the characters of the boys with certain animals for example Hooper with the crow and Kingshaw with the deer or the moths. However the main purpose is to give insight into the boys’ characters by the way that they respond to the different creatures that they meet.
It may not at first seem as if the creatures in this novel are particularly important to the story, it is after all about two boys, one of whom persistently persecutes the other and ultimately causes his death, however on closer analysis the animals chose do have an important part to play whether as symbols of the characters of the boys, or as symbolising the central theme of the story, the nature of the relationship between hunter and hunted, or predator and prey
Actually this conclusion would be equally good as an introduction.
Be general and wide-ranging in your intro, make generalised statements, give an overview, a summary without details.