Flight is the primary mode of locomotion used by most species of birds. Flight can assist birds with feeding, breeding, avoiding predators and migrating.
Our early ancestors must have not only been fascinated but also envious of a bird's ability to fly through the air with ease and grace. A bird's adaptations for successful flying include a high metabolic rate, feathers that are locked in a lattice-like array and low body weight.
The low weight starts with a bony skeleton that usually weighs less than the entire complement of all of the bird's feathers. For example, the skeleton of a frigate bird, with its 7-foot wingspan, weighs only 4 ounces, which is less than all its feathers weigh. The bones of a bird are hollow, braced by internal struts and honeycombed with air sacs. The sacs are connected to the lungs so that during flight air flows through them, speeding the supply of oxygen to body tissues for increased metabolic activity.
Each bird species has a bill or beak of a particular size and shape that is an adaptation to aid a certain life style. In order to help keep weight at a minimum, this bill is not made of heavy bone, but instead it is composed of very strong but lightweight horn. In addition, the bill does not contain any heavy teeth to add extra weight.
Birds also don't have a bladder and urinate as soon as it has to getting rid of the useless weight. All of these particular design strategies produce a bird that can help resist forces that work to keep it from efficient flying, gliding or soaring. For example, the large golden eagle can have a wingspan of up to 7.5 feet and yet weigh in at a total of less than 8.8 pounds.