Albert Barnes Commentary Isaiah 31:5

"As birds hovering, so will Jehovah of hosts protect Jerusalem; he will protect and deliver [it], he will pass over and preserve [it]." — Isaiah 31:5 (ASV)

As birds flying - This is another comparison indicating substantially the same thing as the former: that Yahweh would protect Jerusalem. The idea here is that He would do it in the same manner as birds defend their young: by hovering over them, securing them under their wings, and leaping forward if they are suddenly attacked, to defend them.

Our Saviour has used a similar figure to indicate His readiness to have defended and saved the same city (Matthew 23:27), and it is possible that He may have had this passage in mind. The phrase ‘birds flying,’ may denote the rapidity with which birds fly to defend their young, and therefore, the rapidity with which God would come to defend Jerusalem; or it may refer to the fact that birds, when their young are attacked, fly or flutter around them to defend them; they will not leave them.

And passing over - פסוח pâsoach. Lowth renders this, ‘Leaping forward.’ This word, which is usually applied in some of its forms to the Passover (Exodus 12:13, 23, 27; Numbers 9:4; Joshua 5:11; 2 Chronicles 30:18), properly means, as a verb, to pass over, and therefore, to preserve or spare. The idea in the passage is that Yahweh would protect Jerusalem, as a bird defends its young.