Most terns hunt fish by diving, often hovering first, and the particular approach technique used can help to distinguish similar species at a distance. Sea terns often hunt in association with porpoises or predatory fish, such as bluefish, tuna or bonitos, since these large marine animals drive the prey to the surface. Sooty terns feed at night as the fish rise to the surface, and are believed to sleep on the wing since they become waterlogged easily. Terns of several species will feed on invertebrates, following the plough or hunting on foot on mudflats. The marsh terns normally catch insects in the air or pick them off the surface of fresh water. Other species will sometimes use these techniques if the opportunity arises. An individual tern's foraging efficiency increases with its age.