National Geographic : 1971 May
to avoid detection by the condors, for then they lack the updrafts they need for flight. Aerodynamically, the con dor has evolved a delicate balance between body weight and wing size. This ratio, a marvel of nature's working, governs how-and when-this giant carrion eater flies. The larger the bird, the greater proportionately must be the wingspread required to support it in soaring flight. For his 25-pound body the Andean condor has a ten-foot spread that blots out almost 20 square feet of sky. He is one of the biggest of birds that fly. The condor rides the air like a glider. He can flap his wings, but not for long. To do so would require larger breast muscles, and thus a heavier body. Free Ride to 15,000 Feet on Rising Currents When the condor does flap his wings, it usually is in bursts of three, four, or five strokes, with pauses between. He reserves flapping for an added boost while crossing an area that has no updrafts, for taking off or landing, or for use in emergency situations. He prefers not to flap; he'll walk a hundred yards from a feeding site just to get to a ridge crest or hillock where, with a leap, he can launch himself on outstretched wings. In flight he moves silently in vast circles or descends in long, straight glides, the wind in his wings becoming audible at a hundred yards or so. Almost imperceptibly his broad tail or the slender, sensitive "fingers"-primary feathers-on his wing tips shift to feel the air and control precisely his direction and speed. He rides air currents to altitudes of more than 15,000 feet and can sail steadily at speeds averaging 35 miles an hour (right). This dependence on currents aloft restricts the condor's range to rugged mountain terrain where steep slopes as sure strong flows of air and to regions where ocean breezes meet coastal cliffs and hills and sweep upward. Before the morning sun rises enough to heat the land and power such wind machines, the big bird remains at his roost. That first condor to visit our blind was an adult male. Sex of the Andean condor, unlike that of its North Amer ican counterpart, the California condor, can be deter mined easily in the field. Males carry a fleshy red or black crest. The adult female is smaller and has a red eye, while the adult male has a light-brown iris (pages 688 and 689). Like a feathered sailplane, a female condor soars above Rio Pasto canyon in Colombia. The wind whines through her pinions as she rides a warm updraft; the birds rarelyflap their wings except on take-off, in landing, or in emergencies. "The condor relies on thermal currents as cars do on gas stations to get from place to place," says the author. He clocked its air speed at an average 35 miles an hour. Perched on a 2,000-foot cliff, Mrs. McGahan used a telephoto lens to photograph this bird as it glided 200 feet below her. 692 KODACHROME © N.G.S.