Adult loons are highly specialized for living in an aquatic environment, swimming and diving to feed on a variety of fish and crustaceans. Their bodies are streamlined and their legs are laterally flattened, characteristics that reduce resistance when swimming under water. Their bones are very dense, enabling them to dive readily. However, to compensate for the added weight of a high bone density, loons must flap their wings continuously when flying to remain airborne.
Their large webbed feet are also used for propulsion and to make quick turns while diving underwater. Their legs are set so far back on their heavy body that they are virtually incapable of walking on land. Loons only come to land to breed, incubate their eggs, or when they are gravely sick.
Loons require a long stretch (almost a quarter-mile!) of water to run on before taking off into the air. If a loon has the misfortune to become "iced in" during a sudden cold spell, or lands in a field, road or parking lot during rain, fog, or a bad storm, it may be incapable of taking off, despite being perfectly healthy.