Because thou hast been my help,.... Or, "that thou hast been my help" ; and so the words may be considered as the subject of his meditation in the night watches, at least as a part of it; and as what gave him a great deal of pleasure to reflect upon, how the Lord had been in times past a present help to him in time of trouble;therefore in the shadow of thy wings will I rejoice; meaning under the protecting power of God, where he knew he was safe, and therefore had reason to rejoice.
The allusion is to the chirping of chickens under the wing of the hen; see Ps 57:1. The Targum is,"in the shadow of thy Shechinah will I rejoice;''referring it may be to the Shechinah, or presence of God, between the cherubim, whose wings overshadowed the mercy seat.