"What lips my lips have kissed" by Millay, identify the personification in the lines 9 and 11. What effect does this use have?

The personification in this lines is, obviously, that of the tree and the birds.  A tree, as an inanimate object can of course know nothing about the birds that have sat on it and it can not miss them when they are gone.  In these lines, though, the tree is given the ability to do so.

The effect here is simply to intensify the poems mood of loss and nostalgia.  It implies that the speaker, like a tree, has simply stood still, moving from summer to winter.  Meanwhile, those people, events, and feelings that made her sing in the summer have gone off like birds leaving a tree.

So the use of this personification intensifies the poem's feeling of the loss of days and people and feelings that have gone and will never again be felt or known in the same way.