Gliding or Non-Flapping Flight

Many soaring or gliding birds appear to hang in the air effortlessly, gaining height with barely a twitch of a wing. These are birds like Vultures, large raptors, Pelicans and Storks with a high lift to drag ratio. Essentially this means that their wings generate a lot of lift without producing much drag. Large birds have evolved to be gliders partly because gliding becomes easier the larger your wings are and obviously small birds cannot have large wings. Secondly, the mechanics of flapping flight become harder to attain the larger you get. This is all related to the fact that mass increases far faster than length. A bird twice as long as another is on average 4 times as heavy. There are other physical constraints to do with the musculature needed to actually flap the wings and the strength needed in the bones to withstand the stresses that these muscles generate. Remember, 'for every action there is an equal and opposite reaction'. Understanding physics is important in biology, however, if all this is a bit complicated just remember that big birds with big wings make better gliders than small birds with small wings.

As an example, Fulmars are excellent flyers and can be seen as they soar around the sea cliffs of Devon. They are excellent gliders and can ride right up to their nests on their stiff unmoving wings. However, Vultures are much much better. A Fulmar in still air can glide quite efficiently, losing only 1 metre in height for every 8.5 metres it travels. The American Black Vulture, however, can glide 22 metres for the loss of only 1 metre in height. This means that an American Black Vulture 501 metres up in the air can, if the air is still, glide 1.1 kilometres or 2/3 of a mile before he hits the ground. The air is seldom still however and wind changes everything, facing in to the wind you get lift without doing anything but go nowhere while travelling with the wind you get fast forward movement but loss of lift. Physical obstructions like cliffs, mountains and large buildings all cause disturbances in air movement, including updrafts of air. Over some lands the air is heated by reflection and radiation from the sun-heated earth. This produces the thermal updrafts that many of the large birds mentioned earlier use to keep themselves aloft.

Over the sea, large physical objects and thermal updrafts are very rare. Instead, Albatrosses and their kin use small local updrafts caused by the wind meeting the waves. These updrafts are small and temporary, so sea birds fly close to the sea's surface, often riding along one wave catching the air that rises over it before switching quickly to another. In this way, their flight is a zigzag from one lot of rising air to the next.