How do birds like vultures, kites, eagles etc fly without flapping wings?

Vultures, kites and eagles do not pick and eat grain like pigeons or even insects so it is not necessary for them to remain at the ground level. On the other hand flying higher up in the sky steadily is essential because only after a thorough scrutiny of a large area they may find some food. For example, a carrion vulture may not see a dead animal unless it has carefully examined hundreds of square kilometers grounds. The reason is only a small number of animals die natural death in scrub or forest.

The case of a hunting eagle or kite is somewhat different. Flying at low level above ground it may miss prey like snake, lizard, squirrel, rabbit, partridge, woodcock etc. hidden in bushes or in some ruins not being able to see it due to obstacles. Sometimes prey becomes forewarned if the predator is nearby. It is also difficult to search a large area when flying close to ground so an eagle or a kite prefers to keep the field of vision as large as possible by flying high. Study the table give above which correlates altitude of flight with the range of vision.

There are two methods by which birds can fly effortlessly: Gliding and soaring. Talking about vulture, when it is gliding in the air its wings are outspread but not flapping. However, as it glides in the air it goes on losing height at the ratio of 15:1, that is, it loses height at the rate of 1 foot for every 15 feet distance covered. In fact, gliding means losing height when flying. A bird can not remain airborne for long when gliding. It has to create force of lift by flapping wings ultimately.

Soaring is entirely different pattern of flying. Here the force of lift is created naturally. Upward flowing air current helps the bird in flying. Air does not become uniformly warm during the day over an area where a town is situated in the midst of lush green surrounding countryside. Roofs and house, roads and open spaces become warmer during sunny day. Air above the town becomes warmer and lighter. Such air rises up and helps a bird to maintain its height on the contrary a bird can gain higher altitude by merely banking, i.e. by raising one wing higher than the other wing. In order to keep flying for hours without flapping wings a bird has to fly within the confines of upward flowing warm air current. Therefore, a soaring bird is seen circling within confined area.

Additional reading:
Bird flight (Wikipedia)
Flying and gliding animals (Wikipedia)

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