In a lively way it wanders from one flower to the other in search for food. It moves forwards and backwards, zigzags, stops and accelerates abruptly, hovers, maneuvers sideways, and even turns upside down. Always beating vigorously. These distinctive features of hummingbirds are due to a bone and muscular structure which are unique to them. The humerus is reduced, so the hand occupies most of the wing. The wrist and elbow joints are almost locked, which results in an almost rigid wing. The specialized shoulder joint allows this atypical wing to rotate freely producing a complete stroke which draws a lying eight. It has eight ribs instead of six thus increasing stability. The muscles also present differences both in volumen and in composition. The two principal flight muscles attach to the keel, which is bigger in relation to that of other birds. A bigger keel implies bigger muscles. These are highly developed and formed only by red fibers, the necessary source of energy to perform this type of flight.