Evolution of the bird

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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

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.

Flight

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.

List of evolutionary changes enhancing flight

Aspects of evolutionary change in birds that enhance their ability to fly are:

1. Air sacs and pneumatic bones – the bronchi of the respiratory system extend beyond the lungs into air sacs which in turn extend into many long bones. These air sacs:
   • reduce the weight as an aid to flight;
   • help the bird to change its centre of gravity as it changes posture from the fairly upright when not flying to the generally horizontal when flying;
   • provide volume in the respiratory system for the large volume of air to provide the oxygen necessary for the energy required for flight.
2. Major parts of the vertebral column are fused to provide a strong, rigid framework for the large muscles involved in flight to work against, for example, the bones of the lumbar and sacral regions are fused.
3. The head is attached by a long, flexible, ‘S’ shaped neck as a means of:
   • adjusting the centre of gravity during flight;
   • reducing concussion on landing to protect the brain and nervous system;
   • facilitate feeding from the ground – this would be otherwise difficult because of the rigidity of the body.
4. The extremely deep sternum or breast bone for the attachment of the large muscles that in flight raise and lower the wing.
5. The provision of a very simple but highly efficient digestive system that, while being light, will support the high metabolic rate.
6. The streamlined shape of birds:
   • generally rounded for better aerodynamics;
   • folded skin to minimise angles and to eliminate air pockets during flight;
   • legs are usually folded up against the body to minimise their effect on airflow.