The poet moves from the meadow to the forest, which he compares to the ever-growing Noah’s ark. The trees also become symbols of Thomas Fairfax’s and his wife Ann Vere’s genealogical trees, now growing together. When you approach the forest, you actually cannot see the trees for the forest because they grow so close together. But when you enter, it becomes spacious like a Corinthian temple. Marvell describes various birds living in the forest: nightingale trying her songs, a pair of turtledoves who are the symbols of marital love (again probably alluding to the Fairfaxes and their happy marriage), having rings on their necks and always mournfully cooing. Then he manages to get a political metaphor out of the woodpecker who keeps the healthy trees clean, but when he finds an oak (the traditional symbol of royalty) which is already rotten in the middle, he chooses it for its house and fells it down. I don’t think I quite follow Marvell here: I know woodpeckers nest in the holes they bore in the trees and I can imagine some very determined woodpeckers can cause a tree to fall if it’s already dead on the inside, but there would be no point for a woodpecker to nest in the tree on the ground. But Marvell continues his political metaphor further, arguing that the oak wouldn’t have fallen if it had not fed a traitor-worm, which is now going to serve as food for the woodpecker’s young. I am not quite sure if the worm is Charles I or maybe some other Royalists (Archbishop Laud?) but we all get the general drift: the old system was rotten to the core and had to fall.