Birds evolved from reptiles and are therefore seen as warm-blooded, feathered, flying reptiles. The most commonly held view is that theropod dinosaurs were ancestral to birds and modern flightless birds derived from these earlier flying birds.
Reptiles and birds have many similarities. It is speculated that sometime during bird evolution, environmental temperatures dropped and some reptiles which already possessed a higher metabolism and warm-bloodedness, survived such a harsh climate. This was primarily because they had split scales that enabled them to maintain their internal temperature by trapping a layer of still, warm air close to their bodies. The beneficial split scales evolved into feathers and these reptiles became ancestral birds. The split scales also aided the birds’ ability to maintain balance during fluttering which, in time, ultimately developed into the ability to fly.
Some species of ancestral birds possessed characteristics that were more suited to flight than others, such as the ability to lay eggs and the absence of a urinary bladder, which both aided flying. Reproduction by laying eggs meant that the female did not have to carry the added weight of her offspring during embryonic development and the absence of a bladder meant that urine did not accumulate before excretion, thus lightening the load. In fact, the urine passed by birds is of a pasty nature as they re-absorb much of the water, which also reduces their dependence on water supplies. Theropod dinosaurs, like modern birds, possessed air sacs that were connected to their lungs, which permitted efficient oxygen absorption during both inhalation and exhalation. Many also had hollow air-filled or pneumatic bones linked to the air sacs, which improved respiration and permitted the body to be lighter, which also assisted flight.
Aspects of evolutionary change in birds that enhanced their ability to fly are: