They leave when they want to leave. They know better than us, they have been doing it for a while.<quoted text>If you use hummingbird feeders, take them down September 1.
Otherwise the birds are apt to stay around too late, and starve on the way South.
I've watched them come and go for 10 years. Seems they all pack up and leave at the same time. They left late Sept last year. Had a straggler or two stop by and keep going. Have seen them all disappear overnight in late August. Have seen them come swarming in the Spring by the hundreds when I lived in their migration path. There are not enough flowers that early to support them.
There is a huge area around the Mexican border, and the Gulf of Mexico itself that are not known for an abundance of daffodils or other such flowers. They follow the stream and river beds when the winter runoff starts and the bugs start blooming, and spread out from there. Bugs start decreasing and they bug out.