Genesis 1:20 (ESV)

20 And God said, “Let the waters swarm with swarms of living creatures, and let birds fly above the earth across the expanse of the heavens.”

On the second day, God separated the water on the earth from that in the air. Now the day has come when the air and the land are populated. The sky must not remain empty and silent. Life needs to be seen and heard. After the Lord makes his voice heard again, the birds appear.