This story is exactly what it proposes: it is an allegory explaining technically how the Holocaust came to occur, using forest animals as metaphors for different parts of society. But that is interesting because in a way, the metaphor is more technical than it might seem. Although the animals in the story might seem completely disconnected from humans, that isn't the case. Technically, the Holocaust literally did happen to animals, and that's the real tragedy, because the premise shows clearly that the novelist believes all of nature could be in harmony.
The state of harmony is disrupted by the Terrible Things. In this case, the Terrible Things are human beings and Nazis. Interestingly, the Terrible Things' decision to execute the birds was well-received in animal nature, but not because of anything in earnest. In reality, the forest was simply scared and confused, allowing the Terrible Things to set the tone for how the animals should interpret their actions.
There are metaphors for economic interests and social establishments, and there are metaphors for betrayal, which means that this book is a full-out attempt to exhaustively detail where complacency fits into the question, because the Holocaust did happen. It literally occurred, and humans should constantly try to understand it, because there is no reason to suspect that if humans can become so complacent as to help Nazis to kill an entire race for no real reason, that they wouldn't be able to do it again. The Terrible Things are not just Nazis, it seems, but something more—they are a force of nature that must be constantly dealt with, lest they get what they want out of the forest, which in this case was actual murder of more than 6 million Jews.
