Group Of Birds Flying Formation At Edge Of Flowing River

A 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.