Bird Flight Over Water

Thursday, June 2, 2011

Nature Notes
by Bob Thomas

One of the most fascinating areas of study is the realm of functional morphology, especially as it relates to animal behavior and adaptive physiology.

That may seem like a mouthful, but to a naturalist trying to understand why animals do what they do and how they do it, it is a font of discovery that usually results in saying, “Now that is really cool!”

If you love watching brown pelicans flying over water in and about America’s WETLAND (coastal Louisiana), you’ve no doubt noticed how often they zip along near the surface. This may occur over smooth water surfaces, or in troughs between waves breaking toward a beach.

Most people who just sit on a beach and watch pelicans glide by don’t notice the relationship. Why would they? They are simply birds flying by, and sometimes they are near the water and sometimes they fly higher.

As one comes to better understand animals in their natural habitats, it may become apparent that everything they do has a purpose. It is fun to see them do something and ask, “Why are they doing that? Might they be gaining an advantage?”

Brown pelicans are large birds, having a wingspan of over seven feet, tip to tip, and weighing 10 lbs. To any animal, efficiency of movement has physiological value. Flying more efficiently saves energy, thus requiring less food and less time feeding.

In fact, brown pelicans fly close to the water, as do other birds, to take advantage of a concept of physics called the ground effect, or sometimes compression gliding.

Ground effect comes into play when the bird is within its full wingspan of the surface of the water. As the bird nears the surface, the efficiency increases. It has everything to do with the relative length of the wings, and it is commonly seen in high aspect ratio winged birds (those with wide, narrow wings) like skimmers, petrels, albatrosses, shearwaters, cormorants, and others.

Basically, as the bird glides over the water the air is “funneled” between the lower surfaces of the wings and the upper surface of the water. The air is thereby compressed and functions like a cushion of dense air that supports the bird aloft, in addition to the normal aerodynamic forces at work. As the bird nears the water surface, the ground effect becomes stronger. It is also more efficient over calm (flat) water.

This aerodynamic phenomenon is very important to aerial wildlife, and it has been copied by humans. During World War II, long-range bombers often flew close to the water’s surface to conserve fuel. Inexperienced pilots coming in for a landing are often surprised as they gradually drop down as expected, then get within half a wingspan to the ground and are suddenly buoyed upward by the ground effect. It even happens in commercial aircraft. Pay close attention when you are on a landing plane and you may feel an unexpected buoyant sensation just before touchdown.

Pelicans prefer to glide along the surface, but must occasionally gain a bit of altitude in order to flap their wings so they can gain speed and resume their glide. Yes, we may often see the relatively large pelicans flying much higher above the water, but we don’t see the physiological tax they pay for escaping the ground effect.

The ground effect also comes into action when the large birds want to land. Pilots often joke that successfully landing an airplane is just a pilot controlled crash. For the pelican, the ground effect allows the bird to slow its flight while remaining aloft until its landing gear (feet) touch down.

In case you wondered, the ground effect works as well over land as it does over water, but over land there is a higher probability of encountering a rock, tree, cliff, building, telephone pole, or the like. Of course, at sea there are buoys and boats!

Bird flight is complex and fascinating. Maybe that is why books and reams of articles have been written about it, and we learn more every day.