5 “He shall defend you between His shoulders, and you shall hope under His wings”. He says this, that your protection may not be to you from yourself, that you may not imagine that you can defend yourself; He will defend you, to deliver you from the hunter's snare, and from an harsh word. The expression, “between His shoulders,” may be understood both in front and behind: for the shoulders are about the head; but in the words, “you shall hope under His wings,” it is clear that the protection of the wings of God expanded places you between His shoulders, so that God's wings on this side and that have you in the midst, where you shall not fear lest any one hurt you: only be thou careful never to leave that spot, where no foe dares approach.
If the hen defends her chickens beneath her wings; how much more shall you be safe beneath the wings of God, even against the devil and his angels, the powers who fly about in mid air like hawks, to carry off the weak young one? For the comparison of the hen to the very Wisdom of God is not without ground; for Christ Himself, our Lord and Saviour, speaks of Himself as likened to a hen; “how often would I have gathered your children,” etc. That Jerusalem would not: let us be willing....If you consider other birds, brethren, you will find many that hatch their eggs, and keep their young warm: but none that weakens herself in sympathy with her chickens, as the hen does.
We see swallows, sparrows, and storks outside their nests, without being able to decide whether they have young or no: but we know the hen to be a mother by the weakness of her voice, and the loosening of her feathers: she changes altogether from love for her chickens: she weakens herself because they are weak. Thus since we were weak, the Wisdom of God made Itself weak, when the Word was made flesh, and dwelt in us, that we might hope under His wings.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)