The majority of North American hummingbirds fly south for the winter, a necessity dictated by their specialized diet and rapid metabolism. These birds rely heavily on nectar from flowering plants and small insects, food sources that disappear across northern latitudes with the onset of cold weather. Since hummingbirds cannot hibernate, they must relocate to areas where the climate guarantees a reliable year-round supply of fuel. This annual migration is a survival strategy, ensuring they escape the food scarcity and energy demands of northern winters.
The Great Migration
The journey south is an astounding feat of endurance, particularly for species like the Ruby-throated Hummingbird, which breeds across the eastern United States and Canada. The southward migration begins in late summer or early fall, triggered not by cold temperatures but by the decreasing duration of daylight hours. This change in photoperiod signals to the birds that it is time to depart before food supplies diminish.
Ruby-throated Hummingbirds travel thousands of miles to reach their wintering grounds in Central America, primarily in southern Mexico and the Yucatan Peninsula. The most dramatic part of this migration is the crossing of the Gulf of Mexico, a non-stop flight of approximately 500 miles. This marathon flight requires the birds to fly continuously for 18 to 22 hours. While some birds take a less arduous overland route following the coastline, many choose the shorter, but riskier, direct route over the open water.
Fueling the Journey
Before embarking on their flight, hummingbirds must undergo intense feeding known as hyperphagia to build up the necessary fat reserves. Fat is the most efficient fuel for long-distance flight, providing more than twice the energy per gram compared to protein or carbohydrates. A hummingbird can nearly double its body weight, gaining 25 to 40% of its normal mass in a matter of days. This added fat is enough to fuel a non-stop flight of up to 600 miles.
Hummingbirds are homeothermic animals with the highest mass-specific metabolic rate of any endotherm, requiring them to constantly consume food to support their high energy expenditure. To manage this demand during the migration, they employ a survival strategy known as torpor. Torpor is a short-term, hibernation-like state where the bird significantly lowers its body temperature and slows its metabolism by up to 95%. This mechanism is used to conserve energy overnight or during rest stops, preventing them from starving.
During the nesting season, hummingbirds only enter torpor when their fat reserves drop below 5%. However, during the late summer fattening period, they strategically increase the frequency of torpor even when their fat levels are high. This change in behavior allows them to conserve the energy gained from hyperphagia, enabling them to stockpile the large fat stores needed for the long migration. The stored fat is not only a source of energy but its metabolism produces water, which helps protect the small birds from dehydration during their long, non-stop flights.
The Non-Migrators
While most North American hummingbirds migrate, there are exceptions, such as the Anna’s Hummingbird, which maintains a resident population along the Pacific Coast. This species is found year-round in areas from California up to coastal Washington and British Columbia, making it the northernmost year-round resident hummingbird. Their ability to overwinter in these northern climates is largely dependent on the mild coastal weather and the availability of non-native, early-blooming plants and human-provided nectar feeders.
Anna’s Hummingbirds, like their migratory counterparts, rely on torpor to survive colder periods. They can enter this state nightly, which allows them to endure extended periods of near-freezing temperatures. The year-round availability of both nectar and small insects contributes to their success as non-migratory residents. The expansion of this year-round residence into northern regions is a relatively recent development, likely facilitated by urbanization and the supplemental feeding from people.