And God said, “Let the water teem with living creatures, and let birds fly above the earth across the vault of the sky.” So God created the great creatures of the sea and every living thing with which the water teems and that moves about in it, according to their kinds, and every winged bird according to its kind. And God saw that it was good. God blessed them and said, “Be fruitful and increase in number and fill the water in the seas, and let the birds increase on the earth.” And there was evening, and there was morning—the fifth day. (Genesis 1:20-23)
The earth is finally finished, and it is time to fill it. God begins by creating the different kinds of birds and all the creatures of the sea. This all happened in a 24-hour day, just like tradition tells us.
A great deal has been written about this, and there is no need to repeat it. A couple of points in the text I would like to highlight. God commands the birds and creatures of the sea to multiply and fill the earth. That means it was not immediately filled. There was likely one pair of every taxonomic family, with enough genetic diversity to evolve into other species; one pair of dogs, one pair of cats, one pair of eagles, etc. The reason I say this is because we later read that Adam named all the animals, and this would be impossible to do in a single day if all the species were there from the beginning.
Another reason is that there was a worldwide flood coming in less than 2,000 years. At that time, God chose specific animals to bring into the ark. These animals needed enough genetic diversity to evolve into the different species we see today. Since genetic diversity is lost over time, these original pairs need to be more genetically diverse than those Noah brought into the ark.
All of these birds and animals were created perfect. They were healthy and robust, and were placed on an earth that was designed specifically for them. There were no predators or diseases to worry about. They were very prolific; none of their young were small or weak. Within a hundred years or so, they could easily fill the entire earth.