Hummingbirds are amazing little birds thatcapture our imagination. Their ability to hoverand fly backwards sets them apart from other birds. These tiny dynamos have almost insatiable appetites, drinking up to twice their body weight in nectar each day from flowers. Another fascinating aspect of hummingbirds is their remarkable yearly migration. Many hummingbird species migrate hundreds or thousands of miles each fall to their wintering grounds and then back again in the spring. This raises the question – what cues tell hummingbirds that it’s time to fly south for the winter?
Day Length
One of the main factors that triggers hummingbirds to migrate is day length. Hummingbirds have internal circannual biological clocks that are attuned to the shortening amount of daylight that occurs as autumn approaches. Scientists have conducted experiments demonstrating that decreasing photoperiods lead hummingbirds to become restless, eat more, and gain fat deposits required for migration. Additional environmental cues such as dropping temperatures may augment the hummingbirds’ response, but the shortening days are the primary trigger. The hummingbirds’ internal clocks guide them to start migration before adverse conditions arrive.
As days shorten in late summer, hummingbirds begin eating more to store fat reserves. Their weight may increase by as much as 40-50%. This extra fat provides critical energy for the long migration journey. The hummingbirds also become more hyperactive and antsy as migration time nears. When the cue of sufficiently shortened day length is reached, the urge to migrate becomes strong. The hummingbirds’ orientation and navigation seem to be linked to the angle of sunlight and changing landscape patterns. As the hummingbird flies along its migratory route, these environmental cues guide its journey southward.
Availability of Food Sources
The dwindling of flower nectar and insect food sources is another factor signaling hummingbirds to migrate. Hummingbirds have very high metabolisms and must consume large amounts of food daily. As winter approaches and freezing temperatures arrive, the availability of nectar-producing flowers and active insects sharply declines.
Hummingbirds depend on nectar as their main food source. In temperate climates, many species of nectar-producing flowers begin to wane and die back as autumn approaches. This necessarily impacts the hummingbirds’ food supply. Scientists have noted that hummers appear to time their migration to coincide with falling food levels. The lack of sufficient nectar sources to meet the hummingbirds’ high energy needs provides incentive to seek warmer southern climates with adequate flowers.
Insects comprise the other important food source for hummingbirds. Small insects like gnats, aphids, spiders, caterpillars and tree sap provide key proteins, fats, minerals and nutrients hummingbirds require to meet their high metabolic demands. As winter nears, the number of active insects declines significantly. This decrease in insect food availability provides another signal to hummingbirds that its time to migrate south.
Advancing Cold Temperatures
Progressively colder fall temperatures serve as another stimulus for hummingbird migration. While some species can tolerate occasional light frosts, hummingbirds are vulnerable to prolonged freezing temperatures. Their small size and high metabolic rate make it challenging for them to cope with extreme cold. Prolonged freezing temperatures would put hummingbirds at risk of starving or freezing to death.
In late summer and early fall, nighttime temperatures begin to regularly dip below freezing while daytime warming continues. This increasing cold serves to spur hummingbirds on their southerly migrations. Scientists have found the timing of colder temperature advancement each year aligns with earlier hummingbird migration start dates. The increasing cold provides confirmation that winter is approaching and it’s time to move on.
By migrating south, hummingbirds can escape freezing winter conditions. They spend the winter in tropical and subtropical climates where adequate warmth, food and habitat allow them to survive until spring. Leaving their breeding range as temperatures drop prevents hummingbirds from succumbing to the cold and starvation. Their sensitivity to cold helps stimulate migration to more favorable southern climates.
Genetic and Hormonal Changes
Research shows that hummingbirds undergo genetic changes associated with migration. Levels of certain proteins and hormones fluctuate to facilitate migration. These changes provide further encouragement to move south.
Studies reveal a marked increase in hemoglobin levels as migration approaches. Hemoglobin is contained in red blood cells and carries oxygen. Higher hemoglobin improves oxygen delivery efficiency and provides extra energy for migratory flights. Scientists also found fat metabolic enzymes increased while enzymes responsible for carbohydrate metabolism decreased. These changes allow hummingbirds to efficiently utilize fat reserves and conserve carbohydrate energy stores.
Additionally, circadian rhythm genes shift to prepare hummingbirds for migration. Circadian cycles regulate daily physiological rhythms. The changes result in increased activity and nighttime restlessness. This circadian shift readies the birds for the long migratory journey. Hormonal changes also occur, particularly increased corticosterone which mobilizes fat and promotes hyperactivity. These internal genetic and hormonal changes complement external environmental cues in stimulating migration.
Advantages of Migration
For hummingbirds, migration provides a number of key advantages:
- Access to adequate food – flowering plants and insects
- Escape freezing temperatures and harsh weather
- Safety from predators who migrate as well
- Reduced energy expenditure needed for thermal regulation in warmer climates
- Better habitat and more abundant nectar sources in wintering grounds
By migrating, hummingbirds can survive the winter and return to their breeding grounds when conditions improve the following spring. They have evolved navigation capabilities allowing them to travel long distances between suitable habitats seasonally. Migration ensures hummingbirds continue thriving despite challenging winters at higher latitudes.
Preparation for Migration
In the weeks leading up to migration, hummingbirds go through important physiological and behavioral changes to prepare:
Eating and Weight Gain
Hummingbirds nearly double their weight by increasing eating substantially. The extra fat provides sufficient energy reserves to complete migration.
Feeding Frequency Increase
As migration nears, hummingbirds start making many more visits to feeders and flowers daily to pack on weight.
Hyperactivity
Hummingbirds display signs of restlessness and heightened activity levels as migration approaches.
Directional Flying
Hummingbirds start making practice flights in the direction they’ll migrate to orient themselves for the journey.
Aggression Increase
Hummingbirds become more aggressive and territorial in order to dominate food sources and eat as much as possible pre-migration.
Chattering
Increased chattering and vocalizations help hummingbirds make contact with others getting ready to migrate.
Migration Route
When the internal and external cues indicate it’s time to migrate, hummingbirds embark on a remarkable journey. Here are some key facts about their migration route:
- Hummingbirds migrate individually, not in flocks like many other birds
- Many species follow a clockwise loop route each year as they travel south and back north again
- They migrate during the day, eating enough to continue flying and resting at night
- Some species fly nonstop over the Gulf of Mexico 500 miles or more
- Most travel between 25-30 miles per hour but can reach up to 60 mph
- Migratory routes are imprinted at a young age and memorized for return journeys
- YOY (young of the year) hummingbirds migrate later, often following slightly different routes
Researchers continue studying how hummingbirds navigate their seasonal migrations. It appears they orient using a combination of geometric coordinates, visual landmarks, stellar navigation, geomagnetic fields and other environmental inputs. These amazing birds are capable of remarkably precise migrations year after year along ancestral routes.
Migration Distance and Timing
Different hummingbird species migrate varying distances between their northern breeding range and southern wintering grounds. Migration timing also varies by species and region. Below are some examples:
| Species | One Way Migration Distance | Migration Timing |
|---|---|---|
| Ruby-throated Hummingbird | Over 500 miles one way | August – October fall migration; April – May spring migration |
| Rufous Hummingbird | Over 3,000 miles one way | July – September fall migration; March – May spring migration |
| Allen’s Hummingbird | Around 500 miles one way | July – August fall migration; March – April spring migration |
| Calliope Hummingbird | Up to 2,000 miles one way | August – September fall migration; April – May spring migration |
As this data shows, migration distances and timing varies widely depending on the species and breeding range location. The longest minimum one way migratory trip for hummingbirds appears to be that of the Rufous.
Route to Migration Destinations
Hummingbirds follow different routes to reach their particular wintering grounds. Here are some examples:
Ruby-throated Hummingbird
Many Ruby-throated hummingbirds east of the Mississippi fly across the Gulf of Mexico to Mexico, Central America and Panama for the winter. Some migrate over land southward along the U.S. Atlantic coast as well.
Rufous Hummingbird
The Rufous hummingbird migrates northward up the Pacific Coast in the spring, and south along an inland route through mountain passes down to Mexico for the winter.
Allen’s Hummingbird
This species migrates south along the Pacific Coast to wintering grounds in Mexico. Some remain year-round in coastal California.
Calliope Hummingbird
The Calliope hummingbird migrates between breeding grounds in the northwestern U.S. and western Canada to winter ranges primarily in Mexico.
Wintering Locations and Habitat
Hummingbirds winter in warmer habitats, mainly concentrated along the Pacific coast and Gulf coast, Mexico, Central America, and northern South America. Some specific wintering grounds include:
- Southern Arizona and California
- Southern Texas and the Gulf Coast
- Mexico
- Panama
- Costa Rica
- Colombia
- Venezuela
They favor tropical and subtropical forests, meadows, parks, and gardens with adequate flowers and insects. Having access to supplemental feeding from humans also helps hummingbirds survive the winter in some areas.
Dangers Along the Migration Route
Hummingbirds face many threats during migration including:
- Exhaustion – The long distance travel over land and water is physically demanding. Fat reserves can become depleted before reaching their destination.
- Starvation – Finding adequate food along the way is critical. Lack of nectar sources mid-journey can be fatal.
- Predators – Birds of prey and other animals prey on hummingbirds during migration.
- Collisions – Hummingbirds can collide with buildings, towers, vehicles, and other structures.
- Severe weather – Storms, high winds, and precipitation put hummingbirds at higher risk of exhaustion and displacement.
- Climate change – Changes in timing of flowers blooming along migration routes threatens food availability.
Research indicates that only about half the hummingbirds that start the fall migration journey successfully make it to their wintering grounds each season. Despite excellent adaptations, the migration trip remains an arduous endeavor.
How Hummingbirds Know When to Return North
In the spring, hummingbirds once again heed cues that signal it’s time to migrate north and return to their breeding grounds. These include:
- Increasing daylight triggers hormonal changes preparing the birds for migration.
- Warmer temperatures mean more food becoming available in the north.
- Internal biological rhythms direct them to begin migration.
- Genetic and physiological changes like increased fat storage that prime them for migration.
Hummingbirds precisely time their spring return to coincide with flower blooming and emergence of insects. This ensures adequate food will be available when they arrive to breed. Their internal navigation system guides them back along the same route to return to their original breeding territory. Different hummingbird species arrive back based on synchronizing with food availability specific to their breeding habitat areas.
Threats and Conservation
Hummingbirds face a number of threats, including:
- Habitat loss from development
- Pesticide use reducing insect food supplies
- Climate change disrupting flowering cycles and migration timing
- Collision threats from structures
- Free-roaming cats preying on hummingbirds
Various conservation initiatives aim to protect hummingbird populations:
- Habitat preservation through parks, reserves and corridors
- Home gardens and green spaces with native plants
- Reducing or eliminating pesticide usage
- Installing collision deterrents on windows
- Citizen science monitoring and reporting programs
- Artificial feeders to supplement food, especially during migration
Ongoing research and monitoring of hummingbird migration patterns allows us to better understand the cues that trigger this incredible seasonal journey. With greater knowledge and committed conservation efforts, we can ensure hummingbirds continue their extraordinary migrations each year into the future.
Conclusion
Hummingbirds are unique and amazing birds with their hovering flight capabilities, rapid wing-beats, and extreme nectar-feeding adaptations. Their epic seasonal migrations allow the birds to flourish across a broad range despite prohibitive winters in their northern breeding zones. Shortening day length serves as the primary trigger that sets migration in motion each fall. Advancing cold temperatures, dwindling food supplies, internal genetic and hormonal changes all reinforce the need to migrate south. Understanding the cues that initiate hummingbird migration gives us insight into their seasonal rhythms and how to provide needed support along their arduous journeys. Ongoing research and conservation efforts focused on hummingbirds will ensure the continuation of their incredible migrations for generations to come. The sight of tiny hummingbirds arriving in spring to sip nectar from bright flowers has captivated people’s imaginations for centuries. Learning more about their migratory patterns perpetuates the wonder and deepens our connection with these phenomenal little birds.
