Jamieson, Fausset and Brown on Isaiah 31:5

5. As in the image of "the lion," the point of comparison is the fearless might of Jehovah; so in that of the birds, it is His solicitous affection (De 32:11; Psa 91:4; Mt 23:37).

flying--Rather, "which defend" their young with their wings; "to fly" is a secondary meaning of the Hebrew word [Maurer]. "Hovering over" to protect their young [G. V. Smith].

passing over--as the destroying angel passing over, so as to spare the blood-marked houses of the Israelites on the first passover (Ex 12:13, 23, 27). He passed, or leaped forward [Lowth], to destroy the enemy and to spare His people.