In "Biblical Perspective" Gordon Snider writes:
Two contrasting images are used here to describe the care of God. He shall cover thee with his feathers suggests the tenderness of His care — the gentleness of His dealings with us. The imagery is that of a parent bird. (Do not miss the connection with the danger to birds in the previous verse.) But gentleness should not be confused with weakness, for He is also thy shield and buckler. Here the imagery is of the hardened warrior in battle. Both items were used to protect the soldier from spears and arrows. The shield was larger and used in stationary situations, while the buckler was smaller and used in mobile, “hit-and-run” type situations. God protects us in battle, whether it be an open assault of the enemy, or guerrilla warfare. He is prepared for either!
Source: Studies in the Psalms: Adult Teacher's Insights, page 59.