The Ruby-throated Hummingbird (Archilochus colubris) is the only species that breeds in the eastern United States, including Northern Illinois. These birds undertake one of the longest migratory journeys relative to their size. Understanding when these travelers begin their southward trek requires focusing on the biological mechanisms that govern their movements, rather than just local conditions.
The Critical Departure Window
The precise moment for migration is dictated by an internal biological mechanism, not by the immediate availability of food or temperature changes. The main trigger is the photoperiod, or the decreasing number of daylight hours. This signals that winter is approaching and causes hormonal changes that initiate restlessness and intense feeding.
For Northern Illinois, the bulk of the local hummingbird population departs within a predictable window that spans from late August through mid-September. While some adult males—the first to leave—may start their journey as early as late July or early August, the majority of birds will be gone before the end of September. Migration movements continue in waves, with the departure period extending into early October for the last stragglers. This biological programming ensures the birds leave the breeding grounds well before harsh weather sets in.
Factors Influencing Individual Timing
While the shortening day length sets the overall departure schedule, the timing for individual birds varies significantly based on age and sex. Adult males lead the way, often leaving weeks before females and juveniles. Since males are not involved in raising the second brood, they commence migration earlier, reducing competition for resources along the route.
Adult females follow the males, and the last birds to depart are the juveniles. These young birds must spend extra time gorging on nectar and insects to build up the necessary fat reserves for their first solo journey south. This late-summer feeding frenzy is known as hyperphagia, a period where the birds feed intensely and almost continuously. Leaving feeders available does not delay this process; the birds are driven by the photoperiod and will only leave when their fat stores are sufficient for the first leg of the trip.
The Energetics of the Journey South
The fall departure from Northern Illinois fuels a physically demanding journey. Most Ruby-throated Hummingbirds winter in southern Mexico and Central America, a destination that requires a flight of up to 2,000 miles. The most challenging segment of this migration is the non-stop crossing of the Gulf of Mexico, a flight of 500 to 600 miles over open water.
To survive this 18- to 22-hour flight, the hummingbirds must nearly double their body weight in fat before leaving. A typical three-gram bird may add up to two grams of energy-rich fat, which acts as the fuel for their journey. This stored fat is efficient enough to power a non-stop flight of nearly 600 miles. Individuals that successfully accumulate these large reserves of fat have a greater chance of survival, as strong headwinds over the Gulf can quickly deplete their energy stores.