Allah says in the Holy Quran:
"Do they not see the birds suspended in mid-air up in the sky? Nothing holds them there except Allah. There are certainly Signs in that for people who have faith." (Surat an-Nahl: 79)
A bird uses great energy when it takes flight because it has to lift all its body with its tiny wings. Yet, once it is up in the air, Allah has made it easy for it to remain aloft without expending too much effort. Birds can fly for a long time by allowing themselves to rely on the wind. This way, they hardly tire since they consume very little energy. When the effect produced by the wind diminishes, they begin to flap their wings again. By means of this feature Allah has given birds, they can cover very long distances and migrate to remote destinations.
It is interesting that birds' legs, so slender and little in comparison to their bodies, can carry their entire bodies. It is amazing that so many muscles, veins and nerves can exist in such a slender leg! If birds' legs were thicker and relatively more bulky, it would be more difficult for them to fly.
Almost all birds sleep on one leg. This does not upset their balance since their body weight is concentrated on this one leg. Allah has created birds with the features that enable them to maintain such a delicate balance.
The eyes of water fowl have been created for clear underwater vision. While we can't keep our eyes open underwater for even 45 seconds, water birds can easily catch insects and molluscs in water as they plunge their heads into it. Since this is the only way for them to feed themselves, they must have clear vision underwater. For this reason, Allah has created a special structure in their eyes, which permits seeing underwater.