Penguins most likely descend from an ancient flying ancestor, but they have traded flying through the air for skills at sea. To fly, most birds must remain very tiny. But with an average length of four feet and weight of between 45 and 90 pounds, Emperor penguins no longer have a light, hollow skeleton. Because water is heavier than air, the Emperors have developed strong, sturdy bones to support the larger muscles needed for swimming. These heavier bones also reduce their buoyancy, helping them to stay underwater more easily.
Penguins use their strong arms to literally fly through the sea.
These arms have broad, flat bones that are fused together much like a paddle. Both the upstroke and downstroke of this paddle are highly powerful, and penguins have such control that they can completely reverse direction in as little as a foot. Sometimes they kick to get more speed while performing this underwater flight . Emperor penguins would never survive their hostile environment in isolation. They typically live in colonies of from 500 to 200,000 pairs, keeping each other warm while the males incubate the eggs, and protecting the very young during severe storms.