"... that I should stop feeding hummingbirds in the fall so that they can begin their southern migration. Is this correct?
A. That's a myth (from Cornell's All About Birds). A number of factors trigger the urge for birds to migrate, but the most significant one is day length. As days grow shorter in late summer, hummingbirds get restless and start to head south, taking advantage of abundant natural food, and feeders where available, to fuel their flight. A few individuals, especially Rufous Hummingbirds and a few other Western species, wander east rather than south; causes for this have not been entirely teased out, but it's not feeders that cause them to wander, and if a feeding station is closed down, chances are that a vagrant hummingbird will wander toward worse rather than better conditions.
We encourage people to keep hummingbird feeders full for several weeks after the last hummer leaves just in case a straggler shows up in need of additional energy before completing the long journey south. One of our own staff discovered an adult female Rufous Hummingbird at her feeder in northern Minnesota on November 16, 2004; that bird remained for over two weeks, surviving a blizzard and temperatures that dropped to just 6 degrees Fahrenheit, before leaving at mid-morning on December 3. That day temperatures climbed to a relatively warm 25 degrees; the bird's chances of survival without the feeder she stopped at were significantly lower."
I thought I'd re-print this Cornell Q&A, as I know folks who either don't feed past September or who quit when they don't see a hummingbird for a day or two. Personally, I think that's a sad mistake. I have spent many a winter-like day, changing out cold, slushy hummingbird feeders for ones I've warmed. Just think how much energy a tiny bird needs to stay warm enough on a snowy day; or how much energy it uses to warm up cold nectar.
Sometimes I won't see a hummingbird for two or three days, then suddenly I'll find several at the feeders. Please keep feeders up till you've not seen a hummer in two weeks. That outta keep even the stragglers safe.
See, if these were my feeders, I'd be alternating; one outside, one inside warming...switch and repeat. I'd keep the snow off the feeders, too, they can hardly find the ports in this clip. Perhaps hang them under an eve or put a baffle above. It's a good idea to keep 'em close to a door, too; you don't want to freeze either. PS: it's a long clip and the end is just like the beginning.
Recipe for nectar: One cup sugar to four cups boiling water. Bring to a boil, cool, refrigerate. During exceptionally cold weather, I use one and a half cups sugar to the four cups water. Stronger than that (say 2/4) some folks say is okay, others say it can hurt their kidneys.