“Then Noah sent forth a dove.”
(Here are the first and second parts of the scholion on this verse.)
As the raven was sent forth before the dove, the Holy Spirit goes forth mysteriously to prepare matter to receive God’s informing Word. Thus, as the Spirit broods like a bird of prey at Creation, prior to God’s speaking, so the raven flies ‘to and fro’ over the waters before the re-creation after the Flood. So, too, does the Holy Spirit overshadow the Virgin Mary to prepare her to conceive the Word Incarnate in her womb.