The ironic thing is that the crows and ravens are there in battle fields to eat the dead but there are too many.
Explanation:
The insistence of the passage here is that there are so many dead in the war and the field is so full of the dead that even those that feed on the dead would be scared of coming out to eat the dead.
This is what is being said here in the language of figures as the crows and the ravens feed on the dead and decadency.
For them to not show up means that the event is that terrible that even they do not want to be around to witness it.
