Why migrate now? RTHU are pollen and insect eaters. As winter approaches this food source is depleted, and the birds would starve if they stayed. Also, during the equinox the daylight and night periods are the same. This equal light darkness situation stimulates chemicals in the RTHU brain that tells them it is time to go south if you want to survive. They will go regardless of the fact that the feeder is still out, but they do have some latitude. If the weather is bad they wait. Southerly breezes are good to migrate with since it requires less energy to reach the wintering grounds.
My experience is that adult males leave first, followed by adult females, then the hatch year birds. If you see an adult male after the local breeders leave it is likely a northern migrant looking for food. Also, the old wives tail that if you leave the feeder out the birds will not migrate is not true. It is that brain-chemical thing that promotes migration - not food source.
The breeders may have gone but please leave out the feeders. Northerly migrants are heading your way, and the extra feeding along the way is helpful. I leave my feeders out until at least December. If we have a hard freeze I bring them at night. I return the feeders predawn since hummers tend to feed early. Leave air space in the feeder so the food can expand if very cold and the feeder does not break. RTHU have been documented into November.
The better reason for leaving the feeders out until December is that is when historically out-of-range hummingbirds arrive. Maryland has six hummingbird species on the state list. I have photographed all six species - Allen's, Anna's, Calliope, Green Violetear, Rufous, as well as the RTHU. These out-of-range hummers normally show up from late October to late December. The District of Columbia has a Black-chinned record, and that could be possible in Maryland. Who knows what other species might visit us. These birds tend to be sub-adults, but adults out-of-range hummngbirds have also been recorded. Rufous is an annual visitor, but the other four species have only one or a handful of records..
If you encounter a late hummingbird (after October 15) please contact me. If is important that the bird be identified to species in order to demonstrate range expansion of the non-breeders. Also, place your feeder so it is easy to see from inside the house. Kitchen and dining room windows are a good location. This way you have a better chance of seeing any late visitors.
If you would like to see some of the the out-of-range hummingbird species documented in Maryland take a look at my website: https://georgejett.net/index.php/Rare-MD-Birds?page=10. There are also some on page 11. I used banders codes for the website: Allen's = ALHU; Anna's = ANHU; Calliope = CAHU; Rufous = RUHU.
If anyone wishes to share this note with anyone else please do so.