Or HaChaim on Genesis 1:20/אור החיים על בראשית א׳:כ׳

1And God said, "let swarm, etc.": The explanation [of which] is that creatures should exist from the waters. And this thing is not constant, but [rather only] at the time of the proclamation of the creation, and those creatures that the waters swarmed stood forever - like the animal species which the earth brought forth [and] which it did not not bring forth except at the time of the proclamation [of their creation] and they stood forever - according to the commandment of God to them, that they should swarm and be fruitful and multiply. And its stating, "swarms of living souls;" these are the fish. And it used the expression, swarm, to distinguish between it and between [that] which flies, since [the fish] have no power except immediately adjacent to the earth, and [that] which flies, flies over the earth and does not need to be adjacent to it; and it is also [able to] climb up to the heavens, as it states, "upon the face of the firmament." And without this proclamation, the birds would not climb up to the heavens.

2And its stating, "the firmament of the skies," the explanation [of which] is the firmament, that is called the skies. And there is no difficulty about [the] words of [the Sages], of blessed memory, who stated (Bereishit Rabbah 6:6), and these are their words, "Where are the sun and the moon? In the second firmament, as it is stated, 'And God placed them in the firmament of the skies.'...It is an explicit verse and the men of the Great Assembly have explained it (Nehemiah 9:6), 'You have made the skies and the skies of the skies and all of their hosts;' where are their hosts? In the second firmament;" [that one should ask,] according to [the] words of [the Sages], of blessed memory, what will they answer about this verse, "and let birds fly on the face of the skies of the firmament."

3And it appears that according to the vowels, this verse becomes different than the verse of "And He placed them, etc;" since here, there is a kamats under the [letter] reish of rakia (firmament) and there, there is a shva under the [letter] reish of rakia. And behold, when it states, rakia with a shva, it is saying that is relational, [and] the explanation [of it] is the firmament of the skies, and when it states, rakia with a kamats, it is not relational to [the word,] skies, [and] its explanation is, 'the firmament and it is called the skies;' and with this, it is not difficult; and understand [it].