I looked in my heart while the wild swans went over.
And what did I see I had not seen before?
Only a question less or a question more;
Nothing to match the flight of wild birds flying.
Tiresome heart, forever living and dying,
House without air, I leave you and lock your door.
Wild swans, come over the town, come over
The town again, trailing your legs and crying!
‘Living and dying’ are  binary opposites . These are huge concepts, juxtaposed against her ‘tiresome heart’; the poet — and maybe the reader — may view life and death as burdensome. 
 There may be biographical significance here. Edna St. Vincent Millay had many lovers and was bi-sexual in an era when this sort of lifestyle was scandalous.  The vicissitudes of her personal life may inform the meaning of this line.
Wild Swans