Symbolism, Imagery, Allegory
The animals on the island are fighting for survival just like Phillip and Timothy. In their behavior, we see echoes of human behavior. Phillip's interactions with these animals become a metaphor for the human relationships we see in the backdrop of War World II. For example, when Phillip invades the nesting grounds of the birds, they attack him:
Wondering what had caused the birds to attack me, I felt around in the sand. Soon, my hand touched a warm shell. I couldn't blame the birds very much. I'd accidentally walked into their new nesting ground.
They were fighting for survival, after the storm, just as I was. (16.32-33)
And when he sticks his hand into the moray eel's hole, it bites him:
Pain shooting up my entire arm, I lay panting on the edge of the pool and gingerly began to feel my wrist. It was bleeding, but not badly. But the teeth had sunk in deep. (17.19)
Just as the animals on the island have to defend themselves against intruders (Phillip), the people of the Caribbean must defend themselves from the invading Germans.