Commonly refers to an organisnm capable of flight using appendages called “wings” to generate lift. They act as a glider
taking advantage of thermals to
increase thier lift and minimise drag.
They are also usually very light, small and
aerodynamic.
Some birds however don’t have the advantage of flight. Some birds, for example the emu, are too big and heavy to fly.
Also birds are NOT mammals, they are AVIANS.