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.