Hollow Bones
In order to fly, birds must necessarily be lightweight. The less heavy they are, the better they are able to attain the necessary lift to fly and to easily propel themselves while flying. One of the adaptations that helps them achieve this is the development of hollow bones. This modification to their skeletal system translates to pneumatic, air-filled bones that are less dense that the bones of other animals. The inside of bird bones looks like a honey comb. The bones of birds are so light that they weigh less than the bird's feathers put together.
Missing Organs
Another factor that reduces the weight of birds is the modification of its internal organs. The anatomy of birds makes no room for anything that is not necessary. Instead of two ovaries, female birds have only one. Birds do not have teeth, but grind their food in the gizzard. Also, the reproductive organs of birds, like the testes, oviducts and ovaries are small for most of the year. They only enlarge when the mating or breeding season arrives.
Light Feathers
Even though feathers are necessary for flight, these feathers have a lot of adaptations that make them lightweight, but strong enough for flight. Feathers are made up of keratin, and the shafts of the feathers are hollow. Birds have six different types of feather, the contour, semiplume, down, filoplume, bristles and powder-down feathers. The wings of birds are modified forelimbs with light flight feathers that enable the birds to fly.
Fused Bones
Birds have a significant number of reduced muscles to help them reduce their weight. The most-developed muscles in birds are the flight muscles that they need to fly. The bones are mostly fused, reducing the need for muscles to hold the bones together. The skulls of birds are very thin and the beaks are very light.