Bird bones are hollow and filled with extensions of the body's air sacs , which are extensions of the lungs that allow for increased respiratory efficiency. The eyes are large and the visual cortex of the brain is well-developed. Other anatomical features of birds that are probably connected with flight are not visible in the skeleton. Birds lack a bladder, and they excrete uric acid in the form of solid white crystals, rather than urea in a water solution. But the most obvious anatomical feature of birds is, of course, their feathers.
Feathers are highly modified scales, and are important in several ways.
Some birds, such as the flycatchers, have bristlelike feathers around the mouth that assist in trapping insects in flight. On the other hand, the females of many species with brightly colored males have rather drab feathers acting as camouflage. For a short lesson on external bird anatomy, try this tutorial .