Hummingbirds are amazing migratory birds that make epic journeys between their summer and winter homes every year. But how do these tiny birds, some weighing less than a penny, know precisely when it’s time to take off on their long migrations?
The Internal Clock
Scientists believe hummingbirds rely on an internal clock or circannual rhythm that lets them know when it’s time to migrate. This innate timekeeping system is sensitive to changes in day length. As days get shorter in late summer, hormonal changes are triggered in the hummingbird’s brain signaling it’s time to prepare for the journey south. This allows the birds to begin eating more to store fat reserves and rest up for their travels.
Research has shown that hummingbirds kept in captivity still recognize when it’s the season to migrate, even without environmental cues. So this internal clock appears to be hardwired into their genetic makeup after thousands of years of migration patterns.
Food Availability
Another key factor is food availability. Hummingbirds time their migration to leave temperate climates before flower nectar and insect populations decline in late summer and fall. The decreasing daylight also causes many flowering plants to begin dying back, which serves as another signal to the birds it’s time to depart.
Some research indicates that hummingbirds may get a sense of when food will become scarce by monitoring the quality and quantity of nectar they are finding in flowers. As nectar levels drop, this triggers their urge to migrate.
Fat Storage
Hummingbirds have voracious appetites because they need tremendous amounts of energy to power their wing beats during flight. In the days before migrating, hummingbirds go into hyperphagia – eating as much as possible to store up fat reserves. During this period a hummingbird may consume double its normal food intake.
They can increase their body mass by 25-40% in preparation for migration. This extra fat provides the fuel they’ll need to fly hundreds or even thousands of miles.
Weather Changes
Shifting weather patterns in late summer are another clue it’s time for hummingbirds to head south. Dropping temperatures, increasing winds and rains signal to the birds that the seasons are changing and colder weather is approaching.
Some species, like Rufous hummingbirds, often begin migration as soon as late summer storms start passing through an area. The arrival of weather fronts prompts them to start heading south to stay ahead of the cold.
Hormonal Changes
In response to environmental cues like decreasing daylight, hummingbirds undergo hormonal changes that trigger migratory urges. Hormones like corticosterone increase appetite and fat storage before migration. Testosterone is also produced to stimulate muscle growth and provide strength for the long journey.
Levels of another hormone, prolactin, rise at the end of summer. This energizes the birds and provides a sense of direction as they begin migrating south.
Solitary Migration
While hummingbirds may nest socially and interact at feeders, migration is a solitary affair. Each individual bird navigates its way over mountains, deserts, and other terrain completely solo. There is no collective signal that tells them precisely when to go.
Instead, each hummingbird leaves independently when its individual internal clock and food supply tells it’s time. This leads to a staggered pattern of migration as birds progressively funnel through an area over a period of several weeks.
Stopover Sites
While some hummingbirds may fly nonstop for hundreds of miles, most break up their migration into segments by stopping to rest and refuel along the way. Oasis-like stopover sites provide the opportunity to feed and gain energy for the next leg of the journey.Good stopover habitat helps sustains hummingbirds on their marathon migrations.
Navigation
Hummingbirds find their way by using several different navigation strategies. Like many migratory birds, they can sense the Earth’s magnetic field and use this as a compass to determine direction. They also rely on the sun for orientation and calibrate their internal clock to the movement of the sun across the sky.
Landscape features like mountains, coastlines and rivers provide familiar landmarks they can navigate by. And some research indicates hummingbirds may even have a star navigation capability using celestial bodies for direction.
Route Fidelity
Remarkably, individual hummingbirds tend to follow the same migration routes year after year. This route fidelity is especially pronounced in ruby-throated hummingbirds. Studies of tagged birds show they not only take the same route south in fall, but follow nearly the identical path on their return trip north in spring.
This ability to precisely retrace a route over thousands of miles is an amazing navigational feat for a bird that weighs less than a nickel.
Length of Migration
Different hummingbird species migrate varying distances depending on their breeding and wintering grounds. Here are some examples of hummingbird migration distances:
Species | Migration Distance |
---|---|
Ruby-throated hummingbird | Over 2,000 miles between Canada and Central America |
Rufous hummingbird | 3,000 miles between Alaska and Mexico |
Allen’s hummingbird | Around 1,000 miles between California and Mexico |
Broad-tailed hummingbird | Over 1,500 miles between the western U.S. and Mexico |
As these migration distances show, hummingbirds travel astonishingly long distances relative to their tiny size. The lesser violetear hummingbird holds the record for the longest migration of any hummingbird – flying over 5,000 miles between Mexico and South America.
Timing of Migration
Hummingbirds don’t all migrate at once, but rather follow a pattern based on geography and species. The timing of when different hummingbirds migrate can be summarized as follows:
Species | Migration Period |
---|---|
Ruby-throated hummingbird | August – October fall migration; April – May spring migration |
Rufous hummingbird | July – September fall migration; March – May spring migration |
Allen’s hummingbird | July – August fall migration; March – April spring migration |
Broad-tailed hummingbird | July – September fall migration; April – May spring migration |
As the table shows, hummingbirds that breed farthest north generally migrate first in fall. So species like rufous hummingbirds leave the northern U.S. and Canada by July. While southwestern birds like Allen’s don’t head south until late summer.
Differential Migration
An interesting fact about hummingbird migration is males and females often leave and arrive at different times. Males tend to migrate earlier in spring and return later in fall in order to stake out the best breeding territories.
Some male hummingbirds even spend the winter along the Gulf Coast so they can arrive on their breeding grounds first when spring arrives. This early arrival helps ensure they’ll secure prime nesting habitat before the females show up.
Threats and Dangers
Hummingbirds face many threats and challenges during their arduous migrations:
- Exhaustion – The physical demands of migration can be fatally taxing.
- Starvation – Finding adequate food at stopovers is critical.
- Predators – Birds of prey and other animals prey on hummingbirds.
- Severe weather – Storms, high winds and cold temperatures take a toll.
- Urban obstacles – Buildings, communication towers, and other structures pose hazards.
Research estimates anywhere from 30-50% of hummingbirds die before completing their first southward migration in fall. Those that survive complete an epic journey demonstrating nature’s wonder of navigation.
How You Can Help
Here are a few ways you can help support migrating hummingbirds on their incredible journeys:
- Provide nectar feeders as stopover food sources.
- Plant native flowers and trees that provide food.
- Avoid pesticides that reduce insect populations.
- Install windows to prevent collisions.
- Keep cats indoors so they don’t prey on hummingbirds.
Amazing Endurance Athletes
The speed and endurance displayed by tiny hummingbirds on their migrations is an extraordinary feat of nature. These incredible journeys are driven by an innate sense of timing and navigation as hummingbirds travel between their seasonal homes.
Research continues to uncover the hummingbird’s secrets of mapping out a migration route, flying hundreds or thousands of miles, and arriving at their target destination. But some mysteries still remain about these extraordinary migratory instincts in one of Earth’s smallest birds.
Conclusion
Hummingbirds have an innate ability to know when it’s time to migrate based on internal circannual rhythms attuned to changing daylight, food supplies, weather patterns and other cues. Their navigational capacity allows them to travel vast distances to winter homes they’ve never seen. And these amazing little athletes do it all fueled only by flower nectar and their tenacious endurance.