Think of the doves and woodpeckers who have a low aspect ratio. They need to get away from predators quickly and make sharp turns. The shape of their wings allows them to do these things. On the other hand, falcons and frigatebirds have high aspect ratios, giving them the ability to sustain high-speed flight for long periods. However, these birds do not have wings that can be flapped as rapidly as their woodpecker cousins who have better maneuverability.
Many birds have hollow bones to reduce their weight.
These special bones are very strong though, to cope with the forces necessary for flight. Instead of relatively heavy teeth, birds have light jaws and beaks. The gizzard, present in many birds and used for digestion, is in the middle of the body, and helps keep the bird steady when flying. Even the reproductive organs are specialized to reduce size. A female bird has only one ovary which is normally very small.