Many birds are excellent swimmers and divers, including such distantly related types of birds as grebes, loons, ducks, auks, cormorants, penguins, and diving petrels. Most of these birds have webbed or lobed toes that act as paddles, which they use to propel themselves underwater. Others, including auks and penguins, use their wings to propel themselves through the water. Swimming birds have broad, raftlike bodies that provide stability. They have dense feather coverings that hold pockets of air for warmth, but they can compress the air out of these pockets to reduce buoyancy when diving.

Many fish-catching birds can dive to great depths, either from the air or from the water's surface. The emperor penguin can plunge to depths of more than 250 m (850 ft) and remain submerged for about 12 minutes. Some ducks, swans, and geese perform an action called dabbling, in which they tip their tails up and reach down with their beaks to forage on the mud beneath shallow water.