וְעֹוף֙ יְעֹופֵ֣ף עַל־הָאָ֔רֶץ
Flyers flying over the earth
As with the swarm, the flock of "birds" is a large general category. The verb form, עוּף (`uwf), is used to designate flying. The root further points to the most common flyer, the birds, due to their using their wings to wrap, or cover, their young. The noun form is עוֹף (`owf), with the pointing of the vowel different.
עַל־פְּנֵ֖י רְקִ֥יעַ הַשָּׁמָֽיִם
The face of the firmament
There has been much discussion on the nature of the firmament and its relationship with the heavens. As we saw on the second day, the firmament is called "heaven". However, many translate this simply as the air. This seems to be the sense of the birds.
But what is the surface of the sky? Where does the air begin?These concepts are defined by the observer.
The air that every living thing on earth and in the sky breathes begins at the surface of the earth. However, the surface of the heavens is to be viewed from below. It is clear that the sky continues well beyond the birds. That extension of the sky is where the "heavenly bodies" reside. With the earth as the vantage point, the face of the heavens is where the sky touches the earth.