What are the special features in the internal structure and external structure of birds associated with flight.

The features in the external structure include the presence of wings, feather cover, streamlined body shape. The features in the internal structure include the presence of hollow tubular bones. They are lightweight, which reduces the weight of the bird. You can also include the presence of well-developed muscles that raise and lower the wing, keel – the outgrowth of the sternum, to which these muscles are attached. Birds have a light beak instead of a massive jaw and teeth. They have large hearts, there are air sacs to ensure the function of double breathing.