Rather than using their feet to swim, as many aquatic birds do, they use their wings that have been modified to form extremely efficient flippers. Their webbed feet are used mainly when swimming on the surface of the water. Their feathers have become very small and waterproofed, overlapping to provide better insulation. African penguins feed mainly on small pelagic fish (fish which swim on the upper layers of the open ocean) like pilchards, anchovies, horse mackerel and herrings. Competition with commercial fishing has forced them to adapt their diet. They now also eat squid and small crustaceans as well. Since penguins are capable of diving considerable depths, up to 35 metres, remaining under water for 1½ minutes, they can reach fish that other birds cannot. On average a penguin will eat about 300 g of fish a day, although this will increase to over 1 kg before moulting or when feeding older chicks.