Why do penguins and other seabirds have waterproof feathers?

Many seabirds rely on waterproof plumage to survive in cold, wet environments. Their feathers overlap tightly, creating a barrier that keeps water away from the skin while trapping a layer of air for insulation. Birds also preen, spreading oil from a gland near the tail to maintain water resistance. This is especially important for penguins, ducks, and gulls that spend long periods on or in water. Without waterproofing, they would lose heat quickly and become much less buoyant and agile.