The motif of birds
The Greeks thought that bird-watching was a way of telling omens. Jim's religious attention to nature and his role as a literal servant, a warden of an estuary, make him into a prophet. His prophetic magic is his ability to notice the birds, and when he sees the crows migrating across no man's land, he remembers that humans are only a small portion of nature, and this too shall pass.
The motif of gruesome violence
One main feature of the story is its blatant depiction of violent imagery. There are many violent ways to die in the trenches, and the novel depicts them as always terrifying, and always gory, and always surprising. That's what war is like, since either side is desperately trying to catch someone off guard—it means that random acts of violence happen very frequently in the novel.
Blood as a symbol of death
By being literally covered in blood, Jim is forced to understand death in full concrete reality. This is never more clear than when Clancy dies. One second, Jim was with his friend, and the next second, the friend is gone in an explosion, and his blood covers Jim. That's a pretty clear image of the tragedy, trauma and terror of death, especially in war.
Jim's final act as an allegory
It's difficult to make sense of Jim's final moments for a lot of reasons. First of all, the prose isn't as concrete as it usually is in the book, turning to a more lyrical, poetic tone. And then there's the problem of Jim's uncharacteristic participation in violence. Perhaps he was simply driven crazy, or maybe there's more meaning than that. If it's an allegory, then the effect of the allegory would be something like this: Jim's final act of heroic valor represents his conviction to be fearless, even knowing that it would bring him death. It would represent enlightenment, because he would have transcended his fear of death. The poetic death that the novel gives Jim is good evidence to interpret his actions allegorically.
Imogen as a symbolic character
As an elderly woman, Imogen's experience of the World War is important, because it shows the counterpoint to Jim's experience. Her point of view is the world's point of view in a way, because she only knows the horrors of the war through report, not by first hand knowledge—and yet the gravity of the news is enough to cause her an existential crisis. Therefore, Imogen represents the way in which existentialism followed the World War in history.
