Or HaChaim on Genesis 6:20/אור החיים על בראשית ו׳:כ׳

1 מהעוף למינהו. From the birds according to their kind. It is peculiar that the birds are mentioned before the mammals in this list. If the list is in order of their respective importance, the mammals should have been mentioned first; if the list is in an ascending order the crawling creatures should have been mentioned first. Why does the Torah use this peculiar order? Perhaps the birds enjoy a certain distinction over the mammals because they multiply so profusely. It is interesting that only the "pure" birds multiply so profusely; the 24 categories of "impure" birds which are not fit for consumption by Jews do not multiply as profusely. We have a tradition (Chulin 63?) according to which 120 varieties of birds were found on the Temple Mount, all of which are sub-categories of the איה, the falcon. At any rate these birds are greatly outnumbered by the "pure" birds. The Torah lists only ten categories of mammals which may be consumed by Jews. Presumably this is the reason the Torah commanded Noach to have the birds enter the ark ahead of the mammals. The difference in the description of the birds as למינהו, and the mammals as למינה, indicates that there were more categories and sub-categories of birds (compare Rashi). The Torah lists the respective categories of creatures, i.e. birds, mammals, and crawling creatures according to their relative mobility.