Skimming: Why Birds Fly Low Over Water
flock of sea ducks, pelicans, or sandpipers skimming low
over the water's surface is a common seashore sight. Far
from shore, shearwaters often closely follow the contours of
the waves, and gaggles of auklets fly rapidly just above the
water. Skimming permits the birds to take advantage of an
aerodynamic phenomenon known as "ground effect." The
patterns of airflow around a wing that is operating close to
a surface are modified by that surface in a manner that
reduces drag, the resistance of the air to the progress of
the wing. Sometimes overloaded airplanes are sometimes
incapable of climbing out of the ground effect even though
they can maintain flight close to the ground.
Thus, everything else being
equal, it is more efficient to fly close to a surface than
far from it. But things are rarely equal, which is why birds
most often tend to take advantage of the ground effect when
the "ground" is water. The ground effect only occurs when
the flying object is much less than a wingspan from the
surface -- and at such an altitude over land a bird would be
continually flying among obstacles, through grass, and so
on. Only water is sufficiently uncluttered to permit such
close safe passage.
SEE: How Fast and High Do Birds Fly?; Soaring.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.