I have confirmed my theory. Green Herons lick their lips. Well, their beaks.
The photograph above shows him doing just that. It was the second Green Heron of the day that I caught doing it. I know they were different birds since we spotted them on different sides of the swamp.
This heron was determined to stalk the same side of the trail as us.
He did catch a few small fish. His perseverance paid off.