According to Cornell University, there's no harm in feeding hummingbirds year-round. You can feed hummingbirds in winter and fall, for as long as the little buzzers remain in your area. You won't disrupt their migration, but you might help them to survive the cold months.
What Triggers Migration
Some bird-lovers mistakenly believe that hummingbirds migrate when they begin running out of food. This is simply not the case-- if hummingbirds migrated only when all their food was gone, they would never make it to warmer climates in time.
It is not hunger that triggers migration, but changes in weather and the length of days. As days grow shorter, hummingbirds develop an irresistible urge to fly south. This happens even when the weather is unseasonably warm and when food is plentiful.
When to Stop Feeding Hummingbirds
The best time to stop feeding hummingbirds is, quite simply, whenever they stop showing up. After you stop seeing them regularly, there isn't much point in leaving the feeders out and maintaining them. However, it's a good idea to leave the feeders up for a week or two after you last see a hummingbird; it may provide food for a lagging flutterer.
If you live in an area populated by hummingbirds year-round, feel free to leave your hummingbird feeders up all year. If the hummingbirds keep coming, you can enjoy your feathered visitors well into the winter months and into the next spring.
Tips for Feeding Hummingbirds in the Winter
During the cold months, you don't have to change your nectar as frequently as you would during the summer. This is because the nectar is as prone to fungal or bacterial contamination, so it won't "go bad" very quickly. However, you do need to deal with another equally problematic possibility-- freezing.
Leave extra space in your hummingbird feeders so that the nectar has room to expand if it freezes. Otherwise, your feeder may crack. Add nectar to the feeder while it is still warm to keep it lukewarm as long as possible. You can also use outdoor heat lamps to keep your nectar moist, or you can keep the feeders indoors and swap them out as needed.
If you continue to have hummingbirds at your feeders during winter, there is no harm-- and some benefit-- in leaving them up.