Hummingbirds generally migrate south from January to mid-May in the spring, and their northern migration can start as early as July but usually begins in late August to mid-September. The exact dates of the migration depend on the length of the journey, the weather and the availability of food.
Hummingbirds migrate individually, rather than in flocks like many other birds. Even two hummingbirds living in the same area may migrate at different times depending on a variety of factors. Male hummingbirds tend to migrate a few days or a week before females do, which helps them establish their territories so they can try to attract a mate. Older hummingbirds tend to migrate earlier than young adults or juveniles. The study of hummingbird migration is still in process, however, because of the difficulty of tracking such small birds.
Other factors that affect hummingbird migration may include storms, which can delay migration. Cold snaps that prevent flowers from blooming or send flying insects into stasis also affect migration. However, there is some evidence that most hummingbirds follow an established route every year, migrating at approximately the same time. Young hummingbirds on their first migration set a route that they often follow exactly for the rest of their lives, unless extreme circumstances occur to change it. Some hummingbirds are so regular that they show up at each feeder on the same day every year.