Birds are the result of millions of years of evolution of prehistoric reptiles, which modified their arms to turn into wings, allowing them to take flight. There are three theories for the development of the current upper limbs of birds: the arboreal, the running and the parental care. The evolution from claw to wing involved important modifications in the structure of shoulders and arms. This infographic from the Ábaco collection is part of The Art of Flying series, which also includes those originally titled: Adapted Arms, The Secret of Success: Feathers, The Tail as a Rudder, So is Gliding Flight and So is Flapping Flight.