Several factors affect hummingbird migration and when exactly these small birds set forth on their long journeys. The most important factor that determines the timing of hummingbird migration is daylight: the amount of daylight and the angle of the sun relative to the bird's location. As the light levels change seasonally, hummingbirds purposefully eat more, increasing their weight by 25-40 percent through hyperphagia, when their hormones change to compel them to gain weight. That extra fat will be valuable energy for their upcoming long flight.
Natural Food Sources: Hummingbirds will move along their migration routes as food sources appear and disappear. The most important food source is nectar-producing flowers, but the availability of insects is also a factor that can influence when hummingbirds migrate and how fast they travel. This is especially true in spring when abundant insects are a key food source for hummingbird hatchlings to get enough protein for healthy growth.
Weather Patterns: Local storms and strong winds can impact hummingbird migration to a minor degree, delaying the birds' journeys by a day or two. During spring migration, hummingbirds may be caught in bird fallouts. During poor fall weather, migrating hummingbirds may hang out in safe locations for up to a week or two to wait for better traveling conditions.
Age: Mature birds often start their migration earlier than juvenile birds. Younger hummingbirds take longer to build up their new strength and maturity before setting forth on the long migration journey. Birds that hatched earlier in the year may migrate slightly earlier as well, compared to broods laid later in the breeding season.
Bird Gender: In some hummingbird species, such as the ruby-throated hummingbird, males migrate a few days before females. This gives the male birds time to establish territories so they can successfully court arriving females at the beginning of the breeding season. They follow the same pattern for fall migration to establish winter territories.
Total Migration Distance: The further hummingbirds have to migrate, the sooner they start their journey. The rufous hummingbird, for example, has the longest migration of any hummingbird species – traveling between Mexico and Alaska – and may begin migrating long before other hummingbird species are traveling.
In spring, hummingbirds may begin flying north as early as January, taking several months to travel to their breeding grounds and arriving by mid-May at the northernmost parts of their range. In the fall, some species begin migration as early as July, though most hummingbirds don't begin their southward movements until late August or mid-September.