Up here the rule of thumb is to leave the feeders out for 10 days after you've seen the 'last' hummer. That way, any stragglers that you've missed will have a diner available, and 10 days should be more than enough time for the stragglers to all make it through. Leaving the feeders up will not
encourage them to stay. The migration instinct is pretty strong. When they need to go, they'll go.
I think the last broods were out late this year. The migration is running a bit late, too--normally, they'd all be gone by now. I think our straggler is a young'un and I'm hoping he's not hurt. He acts a little bewildered sometimes. If he doesn't hurry, though, he's going to get caught by winter, so if he's hurt, he's gonna hafta heal quick or winter over in hazel's basement with her impatiens plants, a nectar feeder and fruit flies for supper