It is generally accepted that birds evolved from reptiles and may therefore be seen as warm-blooded, feathered, flying reptiles. The most commonly held view is that theropod dinosaurs were ancestral to birds, with modern flightless birds derived from earlier flying birds.
Reptiles and birds have many similarities. It is speculated that during the period when birds evolved environmental temperatures dropped. Some reptiles, already possessing a higher metabolism and warm-bloodedness, survived such a harsh climate because they had split scales enabling 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 the ancestors of birds. The split scales also provided other benefits – they aided in maintaining balance during fluttering which, in time, ultimately developed into the ability to fly.
Certain of the reptilian forerunners of today’s birds possessed characteristics more suited to flight than did others. The egg laying habit and the absence of an urinary bladder 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 connected to their lungs, permitting efficient oxygen absorption when exhaling as well as inhaling air. Many also had hollow air-filled or pneumatic bones linked to the air sacs, improving respiration and permitting the body to be lighter, again assisting with flight.
Aspects of evolutionary change in birds that enhance their ability to fly are:
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