Hummingbirds, the smallest birds in the world, possess one of the highest metabolic rates among vertebrates, requiring them to constantly feed to sustain their energy needs. This extreme biology makes their annual migration, undertaken by most species breeding in North America, an astonishing feat of endurance. Species like the Ruby-throated Hummingbird undertake incredible long-distance journeys every spring and fall. Their migratory behavior is a complex interaction between innate biological programming and environmental signals.
The Signals That Trigger Migration
The primary cue that initiates the migratory cycle is the photoperiod, the change in the duration of daylight hours. As days become shorter in late summer and early fall, decreasing sunlight triggers hormonal shifts within the hummingbirds’ bodies, signaling the need to move south. This internal biological clock overrides the immediate availability of food sources. These hormonal changes prompt a state of restlessness in the birds. The drop in temperature and the decline in natural flower blooms and insect populations act as secondary signals for departure.
The Incredible Journey
Hummingbirds that breed in the eastern United States and Canada, primarily the Ruby-throated Hummingbird, face a daunting route to wintering grounds in Mexico and Central America. A significant portion of the population chooses the most direct path across the Gulf of Mexico. This Gulf crossing is a nonstop flight of approximately 500 miles over open water, taking 18 to 22 hours depending on wind conditions. They fly alone, relying on their internal navigational system and stored fuel reserves.
Other Migratory Routes
Other species, such as the Rufous Hummingbird, travel up to 4,000 miles from Alaska to Mexico. Some Ruby-throated Hummingbirds opt for a longer, over-land route that follows the coastline, allowing for intermittent rest and refueling stops instead of the over-water crossing. Banding studies show that migrating hummingbirds exhibit high fidelity to their routes, often returning to the same locations year after year.
Fueling the Marathon Flight
The physical preparation for migration is centered on intense feeding, a process known as hyperphagia. During this phase, hummingbirds spend significantly more time consuming nectar and up to 2,000 insects daily to rapidly increase their body mass. The goal of hyperphagia is to accumulate a dense store of yellow fat, the ideal energy source for sustained flight. A typical 3-gram hummingbird may double its body weight by adding nearly 2 grams of fat, sometimes increasing its body fat percentage to over 40%.
The Role of Fat Reserves
Fat is an efficient fuel, providing more than twice the energy per gram compared to burning carbohydrates or protein. Fat metabolism also produces twice as much water, a byproduct that helps the birds stay hydrated during their long flights. Calculations suggest that the fat reserves gained can power a nonstop flight of up to 600 miles, precisely the distance required to cross the Gulf of Mexico.
Species That Remain Year-Round
While migration is typical for most species that breed in temperate zones, not all North American hummingbirds migrate. Certain species, like the Anna’s Hummingbird, are considered resident birds, particularly along the Pacific Coast. These non-migratory populations survive milder winters by utilizing year-round nectar sources from ornamental plants and supplemental feeding. The Anna’s Hummingbird also utilizes torpor, a hibernation-like condition where their metabolic rate, heart rate, and body temperature drop significantly to conserve energy during cold nights. This ability allows them to withstand short periods of low temperatures that would be fatal to other species.