In the quiet tidal backwaters of Delmarva black skimmers are frequently seen gliding over the water surface. They dip their lower mandible (bill) into the water and fly just over the water surface waiting for food to be caught up by that protuberance. Once something (think small fish) is felt in the lower mandible the upper mandible slaps shut against the lower and the meal is served. A very effective adaptation to be sure.
