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
Ephrem The Syrian
After Moses spoke about the lights that came to be in the firmament, he turned to write about the swarming things, the birds and the serpents that were created from the waters on the fifth day, saying, "And God said, 'Let the waters cause living things to swarm, and let the birds fly above the earth.' And God created the great serpents and every living creature with which the waters swarmed according to their kind." [ Gen1:20,1:21 ]
When the waters were gathered, which had been ordered on the second day, the rivers were ordered and also springs, lakes, and ponds were revealed. At the word of God, these waters--dispersed throughout creation--brought forth swarming things and fish from within them: the serpents were created within the abysses and the birds soared in flocks out of the waves into the air. As for the great serpents that were created, although the prophets said that Leviathan dwelt in the sea, Job said that the Behemoth dwelt on dry land. David too, speaking of this beast, says that "on a thousand mountains is Behemoth's pasture land," that is "His place of repose." Perhaps it was after they were created that their places were separated so that Leviathan should dwell in the sea and Behemoth on dry land.