我们时常看到,有些鸟平伸着翅膀,一动不动地在空中盘旋,悠闲自若,并且越盘旋越高,绝无空中翻落之虑。这就是鸟的滑翔飞行。鸟之所以能滑翔飞行是利用地面上升的气流作为动力。在晴朗的天气里,阳光照射在地面上将地面晒热,地面又将空气烤热。特别是没有植被的地面空气,热得很快。热空气体积膨胀,比重减少,迅速上升到高空,形成强烈的上升气流。上升气流作用在鸟的翅膀上,鸟不用扑翼也不会掉下来。如果上升气流强烈一些,还能将鸟往上升。凡是能滑翔飞行的鸟,一般体型较大,翅膀也宽大,可以使较多的上升气流作用在它的翅膀上。如陆地上的老鹰、雕、鹫、海洋上空的信天翁都是滑翔能手。大型鸟身体沉重,每 We often see that some birds stretch out their wings and hover in the air motionlessly, leisurely, and hovering higher and higher, with no fear of falling in the air. This is the gliding flight of birds. The reason why a bird can glide in flight is to use the rising air flow as a driving force. In sunny weather, sunlight shines on the ground and the ground heats the air again. Especially the ground air without vegetation is very hot. The hot air expands in volume and its specific gravity decreases. It quickly rises to high altitude and forms a strong upward flow. The updraft acts on the bird’s wings and the bird does not fall off without flapping. If the updraft is a bit stronger, it can also raise the bird. All birds that can fly gliding are generally larger in size and wider in wings, allowing more updrafts to act on its wings. Land-eagles, eagles, dragonflies, and albatrosses over the ocean are gliders. Large birds are physically heavy, each