The interesting attribute to the Great Egret when breeding is that the facial skin (where the lore and the cere are situated) flashes bright emerald green for only a brief period of time before returning to the pale olive green. At this time the bill also changes to black from usual yellow. The "breeding" bill is actually a "courtship" bill and this brings us to courting.
When courting, they erect their mantle and scapula plumes called ‘aigrettes’. This is where the word egret is derived from.
The "breeding" bill is actually the "courtship bill"