There is a science about why birds fly close to the surface of the water. It is called the Ground Effect and applies to the land as well as water.
I will try to keep it as simple as possible for all to understand;
As a bird flies, the air that flows under its wing collides with it and then washes downward. This then pushes the bird upward (actually the wing is pushed up).
Black-winged Stilt demonstrating ground effect
So when birds fly close the the surface of the water there is less room for downwash to disperse. Because of this, it squeezes the air between the wing and the water and creates a buffer of even higher pressure that increases lift. See, it's simple!
European Avocet just above the water
There is more to it, regarding the reduction of drag. This is due to vortices that are generated at the birds wing tips. Higher-pressure air below their wings, push up into the region of lower pressure above their wings and create swirling vortices, which increase drag.
When birds fly close to the water, there is less space for these vortices to develop and it decreases the effect of this drag. All well and good so far I hope.
Recently Cyprien de Sepibus, a PhD student at the EPFL in Switzerland has been studying the flight of Brown Pelicans. He has concluded that the very pronounced arch on their long wings mean that the Pelicans are too far away from the water to benefit from ground effect. This makes us ask the question why do they fly so close to the water? It has to be due to other effects we have yet to determine. Read here if you would like to see the article.
Common Sandpiper, almost cruising above the water
I hope this has been of some interest, don't let the long winged Pelican make you believe Ground Effect is not relevant to all birds. We have so much more to learn.
If you really want to get into the detail, read "Avoidance of headwinds or exploitation of ground effect—why do birds fly low?" by James Finn, Jens Carlsson, Tom Kelly and John Davenport. It is available on JSTOR - Journal of Field Ornithology.
The article is also downloadable here. You will need a general understanding of physics!
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