Surface-feeding ducks, on the contrary, spring directly into the air with one tremendous impulse that gives such impetus that they fly without appreciable pause. Grebes rise with difficulty • except from the water. From a smooth, hard surface they can some times take off in flight, but in herbage they are unable to rise, as the slightest obstructions break their momentum.
There is a considerable group of birds that in their active moments are so constantly in the air that they. may almost be termed aerial creatures. The swallows are an excellent example of these as they secure their entire supply of food in the air, and for hours on end circle and swing with tirelessly moving wings, only perching when their appetite is satisfied, for the purpose of rest at night, or at their nests.
Aerial existence of a stronger, more placid type, is exemplified by the great vultures and the larger hawks that circle and turn on broadly extended wings above the earth, frequently at great heights. Such birds may soar for hours with only an occasional stroke of the wings, as they use the force of rising or laterally moving air-currents to maintain themselves, the only motion being a constant slight adjustment of the angle of the wings, particu larly at the tips, and of the tail to secure the proper amount of upward thrust to enable them to maintain the desired altitude and at the same time move ahead. The turkey vulture (Cathartes aura), which uses a soaring flight constantly for its progress through the air, is seldom abroad on foggy days when the air is still, but on such occasions remains quietly in its roosts. Many other birds soar merely for the pleasure that this occasions.
Pelicans, cormorants, storks, and screamers, to mention only a few, are seen wheeling for hours so high above the earth that they appear as mere specks in the sky, though these birds search for food near, or on the surface of, water or the ground. Soaring flight of a different kind is seen where gulls glide beside or over a vessel, holding steady position in one place for minutes at a time without movement of the wings. This again is occasioned by steady air-currents that are deflected from the surfaces of the boat at a constant angle.
Hawks and the other soaring birds that have been mentioned that perform their flights relatively high in the air have wings wide in proportion to their length. Albatrosses and other sea birds that travel habitually near the surface of the water have long, narrow wings that when fully extended are more or less of equal width throughout their length, and so are somewhat similar in shape to the wings of a monoplane. These birds progress by
utilization of air-currents induced by wind, and are most com mon in pelagic regions where there are regular winds, and are rare in regions of calm. Their flight is quick and subject to sudden turns, so that the method of progression differs somewhat from the smooth, spiraling turns that mark the soaring of hawks and vultures. As the larger petrel-like birds follow in the wake of ships they bank and turn rapidly with stiffly-extended wings, frequently swinging so that the plane of the wings for a brief space is at right angles to the line of the horizon. It is common to see shearwaters caught in the trough of a wave fly rapidly to the crest and then scale with set wings down the succeeding mov ing slope of water.
Birds like magpies and ducks, that habitually fly long distances, travel with a steady beat of the wings that carries them in a smooth, direct line. Inhabitants of thickets and hedgerows, as sparrows and wrens, progress with a tilting flight in which the short, rounded wings move rapidly for a few quick strokes, and then en pause for a very brief instant, so that the flight is rapidly tilting, or irregular in a vertical plane. Another group of birds, as many woodpeckers, fly in long undulations with a regular rise, during which the wings are stroked rapidly, and a slow descent, during which they are closed and the bird progresses through a combination of its previous momentum and the pull of gravity.
Such ground-haunting birds as the grouse and quails regularly walk or run, and use their wings extensively only to carry them from danger. In these the wings move rapidly, so that flight is swift and is accompanied by a roaring sound. The rapid move ment is maintained for a comparatively short distance when the bird drops to the ground to hide in cover. Flight at high speed is thus maintained for only a brief space. The tinamous (Tinamidae) of South America, birds of grouse-like appearance related to the rheas, like grouse, fly only when pressed. They rise violently and drive rapidly away for fair distances, but are reluctant to rise again, and when forced to fly a second or third time do so with some difficulty. They are so seldom in the air that in violent winds they cannot easily control their direction, and often alight so clumsily that they fall.