Feathers are important adaptations for flight, but also provide insulation against the cold and water. Most males species use feather displays to attract the females during the mating season. Feathers are classified according to size and location. Vanned or contour feathers are bigger and form the exterior layers, while down feathers are smaller and found very close to the skin.
Although wings have a bone structure similar to the upper limbs of other vertebrates and have a humerus, ulna and radius, they are adapted for flight because of their stronger muscles and long feathers. The long feathers that generally cover wings are called remiges. They are classified into primaries, which are longer and close to the tip of the wings; secondaries, which are broader and shorter, and kept closely together during the flight; and tertials, which are even shorter and close to the thorax. In some species of birds, such as penguins, wings are adapted for swimming and are similar in appearance to flippers.
Most bird species have light, hollow bones, which facilitates flight. Also called pneumatic, these bones are especially common in the wings of flying birds. The pygostyle, which refers to the fused vertebrae found in the tail regions, is also a flight adaptation of birds. The pygostyle offers extra support to the muscles and long feathers that form the tail of flying birds.
To replace teeth, birds developed different shapes of beaks, which are adaptations to their specific diets. Birds of prey, such as falcons, have beaks shaped like hooks, adapted for killing prey and ripping up their bodies. Hummingbirds have thin and long beaks, adapted for nectar sucking. Birds that feed by filtering the water, such as spoonbills, have horizontally flatter beaks, while grain-eaters, such as pigeons, have shorter beaks. As birds don have teeth to chew their food, they have developed a specialized stomach, called gizzard, to break down their food.
Birds have developed different feet shapes, which are adapted for living in distinct habitats and to perform specific functions. Hawks and eagles have strong claw-like feet, which they use to capture and kill prey, while ducks and seagulls have webbed feet, which are adapted for swimming. Birds' feet are also adapted for perching in robins, for climbing in woodpeckers, for scratching in pheasants and for running in ostriches and emus.