By mid-September, the ruby-throated hummingbirds you are seeing are likely to be northern visitors passing through. Keep your feeders up at least two weeks after you've seen your last hummer for stragglers or young juveniles that need more time to grow before leaving.
"Males departed as of 09/03/15. Females and juveniles are still here, but in lower numbers. I will leave feeders up for them and migraters heading south. Days are getting shorter now."
David G. Toms River, NJ 09/05/15
Alex S. Northlake, IL 09/04/15