And God said,
Let the waters bring forth abundantly the moving creature that has life, and fowl that may fly above the earth in the open firmament of heaven.
All Commentaries on Genesis 1:20 Go To Genesis 1
Basil the Great
Why do the waters give birth also to birds? Because there is, so to say, a family link between the creatures that fly and those that swim. In the same way that fish cut the waters, using their fins to carry them forward and their tails to direct their movements round and round and straightforward, so we see birds float in the air by the help of their wings. Both endowed with the property of swimming, their common derivation from the waters has made them of one family. At the same time no bird is without feet, because finding all its food upon the earth it cannot do without their service. Rapacious birds have pointed claws to enable them to close on their prey; to the rest has been given the indispensable ministry of feet to seek their food and to provide for the other needs of life. There are a few who walk badly, whose feet are neither suitable for walking nor for preying. Among this number are swallows, incapable of walking and seeking their prey, and the birds called swifts who live on little insects carried about by the air. As to the swallow, its flight, which grazes the earth, fulfils the function of feet.