Soaring Seabirds in Bird Flight

Quite different physical principles underlie the soaring flight of seabirds, such as albatrosses, large petrels, and shearwaters. There are no thermals or updrafts over large bodies of water, only horizontal gusts of wind. The air masses that these gusts drive before them are slowed by friction at the water surface, and their speed is consequently slower than in higher altitudes. These seabirds have developed a type of soaring and gliding that does not depend upon the presence of either thermal or obstruction currents.

Seabirds use the differential wind velocities that occur over the sea for their soaring flight. Albatrosses, for example, take off against the wind, rising without beating their wings, until they lose momentum in the faster-moving layers of air farther from the water surface, at the height of about 35 to 65 feet. The birds then turn in a curve and shoot back to the water surface in an oblique gliding path, gaining so much speed that after turning back again they can once more climb against the wind.

In this way albatrosses cab soar for hours and days at a time without ever beating their wings. Ornithologists call this "dynamic soaring" or "gust flying" --- rising into the gust and falling on the lull. It is this seemingly endless form of flying that allows the albatross to survive, since albatrosses cannot fly in the absence of wind. This is a significant factor in their latitudinal distribution.

Since soaring flight over the oceans places quite different demands on the physiology on the bird tan does soaring in thermals, the wings of soaring seabirds have evolved quite differently from those of soaring land birds. In the continuously fluctuating wind conditions over the sea, with gusts changing in direction from one second to the next, a land bird's wings would be sadly out of place. To utilize the lift from rising air masses, the wings of a soaring land bird need to be broad and to present a large surface area; excessive length would cause difficulties in maneuverability. The large maritime soaring birds have the largest wing spans. The Wandering Albatross takes first place in a wing span of over 11.5 feet from wing tip to wing tip and weighs 17.5 pounds.

Albratrosses are the masters of gust flying. Their wings are quite narrow, the forearm portion is unusually long, and the hand portion, in contrast, is relatively short. The entire wing is built for rigidity in the turbulence of sea breezes, and the primary and secondary quills are relatively short and stiff so that the tips will not flutter. Flutter at the wing tips or at the trailing edges (as opposed to the leading edges) of airfoils vortex formation (violent, whirling air masses) and leads to an uncontrollable loss of energy.

The long forearm on the albatross wing bears as many as 37 secondary feathers, but the number of primaries, at 10, is no greater than in other birds. This numerical proportion alone indicates the great extension in the length of the forearm, which constitutes the greater part of the airfoil.