How birds that dive into or swim in water to find food have waterproof water?

Birds that dive or swim in water, such as ducks and penguins, have waterproof feathers due to a special oil produced by a gland near their tails. This oil coats the feathers, preventing water from soaking in and keeping the birds dry and buoyant. Additionally, their feathers are structured to trap air, providing insulation and aiding in buoyancy when submerged. This combination allows them to effectively hunt for food in aquatic environments without becoming waterlogged.