Hummingbirds are amazing little creatures. Their ability to hover mid-air and fly backwards sets them apart from other birds. Another incredible feat is their migration. Some species travel thousands of miles between their summer and winter homes. This raises the question – how do hummingbirds know when it’s time to fly south for the winter?
Changes in Day Length
One of the main cues hummingbirds use to trigger their southern migration is changes in day length. As days get shorter in late summer and early fall, hummingbirds instinctively know to start feeding more in preparation for the long journey ahead. The initial migration south usually begins when days reach 12 hours of daylight or less.
This consistency in day length helps hummingbirds, even young ones on their first migration, to accurately know when it’s time to take off. The tilt of the earth and resulting shortening daylight hours are the most reliable natural indication that winter is approaching.
Availability of Food
In addition to day length, food availability plays a key role. Flowering plants and nectar-producing blooms start to fade and die off as winter nears. Hummingbirds are very in-tune with their main food source. When flowers become scarce, they know it’s time to seek warmer climates with a more abundant nectar supply.
Ruby-throated hummingbirds, one of the most common in North America, depend on the nectar from spring and summer flowers. As this preferred nutrition fades away, they instinctively know that it’s time to fly off and find areas where they can continue feeding.
Fat Storage
Hummingbirds also prepare themselves for migration by building up fat reserves. As days shorten, they start feeding more frequently. This allows them to pack on extra fat, doubling their body weight in some cases.
With this excess stored energy, hummingbirds are equipped to make the long non-stop flights that migration requires. By the time mid-August rolls around, they have reached peak fat storage and are ready to take off on their strenuous journey.
Temperature Changes
Dropping temperatures are another weather-related trigger. Hummingbirds are sensitive to weather and temperature shifts. As northern climates start to cool in late summer and fall, hummingbirds sense these changes.
Colder temperatures mean less food availability. To escape the frigid weather and find warmer areas with plentiful nectar sources, hummingbirds know to begin migrating south before it’s too late.
Internal Circadian Rhythms
Research has shown that hummingbirds rely on internal circadian rhythms or internal ‘biological clocks’ that are synchronized with seasonal changes. These rhythms let hummingbirds instinctively know when to eat more to put on fat, when to start migration, and when to reproduce.
Their circadian cycles are tuned into signals and environmental cues indicating seasonal progression. Hummingbirds accurately use these internal guides to begin migrations at precisely the best time every year.
Orientation During Migration
Remarkably, hummingbirds are able to locate their wintering sites year after year. First-year birds with no experience also manage to find their desired destination. How do they achieve this navigational feat? Here are some of the ways hummingbirds orient themselves during migration:
Celestial Cues
Hummingbirds appear capable of using the position of celestial objects like the sun, moon, and stars to determine their direction during migration flights. These celestial bodies provide a stable guide.
Landscape Features
Major landscape features like mountain ranges, coastlines, rivers, and valleys may provide visual cues that help guide hummingbirds on migration routes.
Sense of Smell
Research indicates that hummingbirds have a strong sense of smell that might play a role in orientation, especially their ability to hone in on nectar sources. They may use associated scents to help locate preferred wintering grounds.
Magnetic Compass
Hummingbirds seem to perceive Earth’s magnetic field and use it as an internal compass. Changes in this field at different latitudes likely provide directional information.
Mental Map
After completing initial fall and spring migrations, hummingbirds may form a mental map for future travel. This would explain why they are able to precisely return to the same winter and summer sites annually.
Stopover Sites
While some hummingbird species make remarkably long nonstop flights over barriers like the Gulf of Mexico, most migrate in a series of shorter hops. Oases of flowering plants along the migration route provide essential stopover sites where hummingbirds can rest and refuel. These habitats play a crucial role in their survival during migration.
Some common stopover sites used by migrating hummingbirds include:
– Backyard feeders and gardens
– Flowering meadows
– Orchards
– Parks and protected natural areas
– River valleys
These stopovers provide vital energy sources in the form of nectar and small insects. Short term fat storage at each site sustains hummingbirds on successive flight segments. Stopover duration can range from a few days to over two weeks as birds feed extensively to rebuild energy reserves before moving on.
Return Flight North
After spending the winter in warmer southern climates like Mexico and Central America, hummingbirds once again demonstrate their incredible migration abilities by finding their way back north in the spring.
What cues prompt hummingbirds to start their return migration? Here are some key triggers:
Day Length
As winter turns to spring, days start to rapidly lengthen in northern regions. Hummingbirds instinctively recognize these increasing daylight hours. For many species, the migration back north begins when days reach around 12 hours of sunlight.
Warmer Temperatures
Warming springtime temperatures provide another signal that hummingbirds use. As northern climates start heating up, hummingbirds recognize that it’s time to fly north again and take advantage of the improving conditions.
Food Availability
Increasing daylight also equates to plants and flowers blooming again. When hummingbirds notice more flowers and nectar availability along migration routes to the north, they know to follow the progressively blossoming plants.
Internal Clocks
Just as their internal circadian rhythms help initiate fall migration, hummingbirds’ biological timing again plays a role in commencing their journey back north. These innate annual cycles guide them on when to depart wintering grounds.
Compass Cues
The various orientation mechanisms hummingbirds use, like their magnetic compass sense, enable them to return to the precise locations of their previous summer territories and nesting grounds.
Conservation Importance
Hummingbird migration is an awe-inspiring phenomenon. Unfortunately, habitat loss along their migration routes threatens their survival. Expanding urbanization and reductions in flowering plants have led to fewer dependable stopover sites.
Preserving natural habitats through conservation practices is crucial. Providing artificial feeders, nectar sources, nesting locations, and migratory corridors can also greatly aid hummingbirds. Supporting these tiny travelers ensures that their extraordinary migratory journeys will continue.
Conclusion
While small, hummingbirds undertake one of the most incredible migratory feats in the animal kingdom. Their ability to know precisely when to initiate migratory movements to wintering and summer grounds hundreds or thousands of miles away is truly astonishing.
Research has shown hummingbirds rely on a range of indicators from day length, food availability, and fat storage to internal circadian rhythms and celestial orientation cues. These factors all provide vital signals that it’s time to begin migration and guide hummingbirds on their epic intercontinental journeys.
Understanding the migration process is key to supporting hummingbird populations. Preserving natural stopover habitats and establishing artificial food sources are important conservation measures. With a range of threats from climate change to urbanization, sustaining migratory corridors will help ensure the continuation of hummingbirds’ extraordinary migratory cycle into the future.
