It is clear to us today what is a bird and what is not as feathers make it difficult to confuse them with any other living animal. Many other features – such as wishbones and specialised joints in the wings – are also unique. We place birds in a major group called Aves.
However, millions of years of evolutionary change have made the birds we see today very different from their early ancestors. As we move back in time along their evolutionary tree, the line between what is a bird and what is not becomes increasingly blurred.
Special features of living birds.
- forelimbs adapted as wings
- toothless beak with horny covering
- enlarged sternum, most with a keel to which flight muscles attach
- modified wrist (semilunate carpal) to allow folding of wing
- reoriented shoulder to allow ‘flapping’ motion of forelimbs
- wishbone (fused clavicles)
Other features include:
- warm-bloodedness (endothermy)
- hollow bones
- loss of parts of the skeleton, particularly in the hand, foot, tail and skull
- no claws on hands
- pygostyle (fused tail vertebrae or Parson’s Nose)
- restructured pelvis
Feathers are formed by the outermost layer of skin. They are made of the protein keratin, the same substance that forms claws, hair, nails and scales. However, the chemical composition of the keratin differs among each of these.