All birds have feathers, and most birds have hollow bones as well. These two traits essential for flight, a capability that many, but not all, birds possess. Hollow bones allow the bird skeletons to be lightweight, making it easier to generate enough lift to take the bird off the ground. This makes sense because it's easier to make a lightweight object fly in comparison to a heavier object. Birds that can't fly, like penguins and ostriches, do not have hollow bones throughout the body, so they weigh more than their flying counterparts and would struggle to get off the ground. Feathers also play an important role in flight. They generate the lift and thrust necessary for flight and help to control the speed and direction of the flight as well. However, feathers serve other purposes as well. They can be used as a display to attract mates, camouflage themselves from predators, provide insulation and repel water in addition to aiding in the art of flight. Feathers and hollow bones are two key features that allow birds to fly and survive.