Anna’s hummingbirds (Calypte anna) are small, fast-flying birds found along the west coast of North America. They are known for their remarkable ability to survive cold winters despite their tiny size. In this article, we will explore how Anna’s hummingbirds are able to make it through the winter months when food is scarce and temperatures drop.
Overview of Anna’s Hummingbirds
Anna’s hummingbirds are members of the Trochilidae family and are the smallest birds found in North America. They have an average body length of 3-5 inches and weigh 2-7 grams. Males have iridescent magenta throat feathers and crowns, while females have grey-green plumage on the throat and crown.
Some key facts about Anna’s hummingbirds:
- Native breeding range is along the Pacific Coast from British Columbia to Baja California.
- Partially migratory – northern populations migrate south for the winter while southern populations are year-round residents.
- High metabolism – need to eat frequently to power flight and keep warm.
- Primary diet is nectar and small insects.
- Heart rate can reach up to 1,260 beats per minute during flight.
- Only bird species able to fly backwards.
- Aggressive defenders of flower patches and feeders.
Winter Adaptations
Anna’s hummingbirds have evolved a number of behavioral and physiological adaptations that allow them to survive the winter months when food is limited and temperatures are frigid.
Energy Conservation
Hummingbirds have extremely high metabolisms in order to power their wings for hovering flight. They typically eat up to half their body weight in nectar each day. However, to conserve energy in winter, they minimize activity by entering overnight and short-term torpor. During torpor, their heart rate and breathing slows down dramatically, reducing their metabolism by 95%. This torpor allows them to conserve huge amounts of energy.
Cold Hardiness
In addition to torpor, Anna’s hummingbirds are surprisingly cold hardy for their size. Their feathers provide 90% of their insulation, trapping air close to the body to reduce heat loss. They can maintain a body temperature of 104 F even when ambient temperature approaches freezing. Some studies have shown Anna’s hummingbirds withstanding temperatures as low as -18 C when food is available.
Fat Storage
Anna’s hummingbirds prepare for winter by building up fat reserves in late summer and fall. They will double their body fat in preparation for migration or winter residency. This fat provides critical energy reserves when food is limited and enables hummingbirds to survive days of cold weather without eating.
Microhabitats
Anna’s hummingbirds seek out protected microclimates to minimize their exposure to the elements. Dense vegetation, cavities, and south-facing slopes provide shelter from the wind and retain heat from the winter sun. Hummingbirds reduce their heat loss by up to 30% simply by using these favorable microhabitats.
Winter Feeding Adaptations
To survive the winter, Anna’s hummingbirds must adapt their feeding strategies when flower nectar is scarce.
Switching Food Sources
With most nectar sources gone, Anna’s hummingbirds rely more heavily on insects, tree sap, and leftover fruits in winter. These foods provide an alternative source of carbohydrates and essential protein. Hummingbirds will also consume more carbohydrate-rich tree sap in winter to fuel their high metabolism.
Reliance on Hummingbird Feeders
In residential areas, Anna’s hummingbirds increasingly rely on backyard hummingbird feeders in winter for nutrition. Feeders provide essential carbohydrates in the form of sucrose-rich artificial nectar. This ready food source has allowed hummingbird populations to expand their winter ranges farther north.
Feeding Territoriality
Anna’s hummingbirds defend winter feeding territories centered around nectar sources such as flowering plants, sap wells, and feeders. By chasing away competitors, they maintain access to reliable food and minimize their energy expenditure. Males and females establish separate non-overlapping feeding territories.
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Behavioral Adaptations
Anna’s hummingbirds display changes in their behavior in winter that improve their chances of survival.
Migration
Northern populations of Anna’s hummingbirds migrate south in fall to warmer climates. This allows them to take advantage of abundant food resources and reduce their energy expenditure. Some hummingbirds migrate long distances, from Canada all the way to Mexico.
Night Roosting
At night, Anna’s hummingbirds reduce heat loss by entering a hibernation-like state called noctivation. Their body temperature and metabolic rate drops, enabling them to conserve energy. They select sheltered roosting spots such as cavities or thick cover to avoid wind.
Feather Fluffing
Hummingbirds fluff their feathers into a rounded shape in cold weather to trap more insulating air. This behavior raises their effective insulation by 20-30%, helping retain body heat.
Sunbathing
On sunny winter days, hummingbirds will sunbathe to raise their body temperature. Facing into the sun with feathers fluffed allows them to warm up quickly and reduce their energy consumption.
Unique Winter Adaptations
Some of the most fascinating winter adaptations in Anna’s hummingbirds include:
Leg Scaling
In cold conditions, Anna’s hummingbirds are able to constrict blood flow in their legs and feet so they lose less heat through their unfeathered limbs. This allows them to minimize heat loss without danger of frostbite.
Thermogenic Hovering
Male Anna’s hummingbirds perform a courtship display called hover-feeding. By rapidly beating their wings while stationary, they generate heat as well as lift. This allows them to defend territories and attract mates even in cold weather.
Wind-Hovering
Anna’s hummingbirds sometimes exploit air currents and winds to hover in place with minimal wing movement. This behavior allows them to conserve energy in winter when hovering efficiency is reduced due to cold muscles.
Hypothermic Torpor
If deprived of food while trapped in cold weather, Anna’s hummingbirds can enter a state of deep, hypothermic torpor. Their body temperature drops as low as 18 C, with heart and breathing rates severely reduced. This extreme energy saving mode enables survival for several hours in harsh conditions until the weather warms or the hummingbird can reach food.
Conclusion
Anna’s hummingbirds are remarkable in their ability to survive cold winters despite their small size and high metabolic needs. Behavioral adaptations like torpor and migration combine with physiological traits such as fat storage and cold hardiness to enable these tiny birds to thrive year-round across a wide range of climates and habitats. Their flexibility and resilience serve as models of survival for a species that calls the western coast of North America home.