what are the main characteristics of birds?
['Feathers. Feathers are the defining characteristic of Aves, found on every living species of bird and no other class of animal. ... ', 'Wings. All birds have wings, although not all birds fly. ... ', 'Beak. All birds have beaks, or bills, made of a bony core surrounded by a thin layer of keratin. ... ', 'Eggs. ... ', 'Skeleton.']