The last male ruby-throated hummingbird to visit my feeders came on 8/30, the
last female on 9/3. For the last several years, the last sighting has been
from 9/1-9/3, but several years ago I saw one as late as 9/20.
Almost always, the hummers start feeding early in the morning (and are one of
the last birds actively using the feeders in the evenings) - but I've noticed
that the last birds to visit each summer don't come until late in the
morning. My guess is that my residents have already left and the stragglers are
migrants who notice my feeders as they're flying through.
Check out this address for amazing photos of a nesting hummer: