Wear and tear would be my best guess.
The way they forage, swinging their heads back and forth, they must rub a lot of abrasive sand, mud and rocks.
I've seen wore and chipped bills on the outside edges of the Roseate Spoonbills in Florida,USA.
It could be a defensive wound?
Is the a breeding colored adult? This is the best example of the line around the face I've seen.
Hi James - yes Breeding adult with line around the face, strangley I have never seen a juvi or a nest - mind you we are in drought - quite possibly they dont breed during these times.