The Black Skimmer has a most unusual bill among birds, with the lower mandible longer than the upper one. It uses it to skim over water in search of small fish. When the lower mandible senses a prey, the upper one clamps down.
As it hunts for food, Black Skimmers leave amazingly perfect straight lines on the water surface as seen in the following photos. Two days ago, a pair of Black Skimmers put on an aerial show and spent several minutes flying back and forth in front of me. However, they did not seem to catch anything that I could see.
I heard the sound of fish jumping in the water, probably scared by the skimming action, but maybe they were too big for these birds to catch.