Hummingbirds, the smallest birds on Earth, have an extremely high metabolism requiring them to feed almost constantly throughout the day. This intense energy demand is manageable during the warm seasons when nectar and insects are plentiful. The arrival of winter presents a massive challenge, forcing these tiny creatures to employ remarkable strategies to cope with low temperatures and a severe scarcity of food. Their survival hinges on two primary methods: migration or adopting a specialized biological state to conserve energy.
The Annual Journey South
The majority of hummingbirds found in North America migrate to warmer climates in late summer and early fall. Species like the Ruby-throated Hummingbird travel to wintering grounds in southern Mexico and Central America, sometimes as far south as Panama. This migration is typically triggered by the diminishing daylight hours rather than the immediate drop in temperature or food availability.
The preparation for this undertaking requires the birds to enter a state of hyperphagia, where they nearly double their body weight in fat reserves. This stored energy fuels the long flight, which for many eastern populations involves a non-stop, 500-mile crossing over the Gulf of Mexico. This flight can take a single bird between 18 and 24 hours to complete without rest or food. Western species, such as the Rufous Hummingbird, follow a route down the Rocky Mountains, covering distances that can exceed 2,000 miles.
The State of Torpor
For both migrating and year-round resident hummingbirds, the primary biological mechanism for surviving cold nights or periods of extreme food scarcity is torpor. This state is a short-term, regulated hypothermia where the bird dramatically reduces its metabolic function to conserve energy. A hummingbird in torpor can reduce its energy expenditure by as much as 95% compared to when it is active.
During torpor, a hummingbird’s core body temperature can plummet by nearly 50 degrees Fahrenheit, dropping from a normal temperature of around 104°F to below 50°F. The heart rate slows from over 500 beats per minute to fewer than 50 beats per minute. The respiration rate can drop from 245 breaths per minute to less than ten. The bird remains in this state, often clinging motionless to a branch, until it wakes up to feed at dawn. It then rewarms itself by rapidly vibrating its wing muscles, a process that takes about 20 minutes.
Surviving the Cold as Year-Round Residents
While many species migrate, populations like the Anna’s Hummingbird along the Pacific Coast and certain birds in the southern United States remain year-round. These resident hummingbirds must rely on natural food sources and behavioral adaptations to survive the cold season. Their natural winter diet includes small insects and spiders, which provide protein, as well as tree sap that leaks from holes drilled by sapsuckers.
For readers living in overwintering areas, human assistance through feeders can be life-saving. In cold weather, it is beneficial to increase the sugar water concentration from the typical 4:1 ratio to a richer 3:1 ratio. This higher sugar content offers more calories per sip and slightly lowers the mixture’s freezing point. To prevent the sugar water from freezing solid overnight, residents can rotate multiple feeders or use ambient heat sources. Feeder hygiene is also important, requiring frequent cleaning to prevent the growth of mold.