Whereas most birds shed their feathers and grow new ones during a relatively long period of the year, penguins molt all of their feathers and, in some species, even shed the shields from the beak within a short space of time. During the molting period, which may last several weeks, the penguin does not enter the water to feed. The ability to withstand intense cold is one of the penguin's greatest assets. In addition, many penguins have a thick insulating layer of fat under the skin. Some species are better equipped for cold weather than others.
Penguins usually walk or hop and toboggan along on their breasts, pushing with wings and feet.
They swim with great speed and agility. Some species progress by porpoising, which is swimming underwater some distance, emerging in a graceful arc to take a fresh breath, and submerging again. Penguins feed on fish, cuttlefish, crustaceans, and other small sea animals. In captivity the king and emperor penguins normally do not learn to pick up their own food, and after they have been taught to feed by strenuous forced feeding they must be fed by hand each day. Penguins are gregarious birds and are found in flocks even at sea.