Fall can be one of the more exciting times for hummer-watching. There is a sense of urgency and purpose as migrating birds stop at the feeders. It is extremely important to keep the feeders clean and full this time of year.
According to bird-n-garden.com:
"Keeping your feeders up and full will not keep the birds from migrating. Remember, they eat lots of protein in their diets too and that doesn't come from sugar feeders. They need insects! Shortened days, not lack of food, will tell the hummingbirds when to begin their journey. Leave the feeders up at least three weeks after seeing your last bird. This will help migrating birds that pass through your area. It is important to leave your feeders up and full. Migrating birds need all the quick and easy nourishment they can find to sustain the incredible energy demands of migration. It is vital that hummingbirds fatten up for their extended trips, particularly those crossing the Gulf of Mexico."
For more interesting migration facts: http://www.hummingbirds.net/migration.html