Riddle

Imagine fifty ravens at the Tower.
Some twenty archers strumming twenty bows
Shoot thirty of the ravens in an hour.
Another twenty-five, alive, arrive,
But ten are killed before that hour's close,
Along with five that had the first survived.
If for one hour more the arrows fly,
But no more ravens come, nor no more bows,
I ask how often does a raven die?