Bird migration is the regular seasonal journey undertaken by many species of birds. Migration carries huge costs in predation and mortality, including from hunting by humans, and is driven primarily by availability of food. Birds migrate to move from areas of low or decreasing resources to areas of high or increasing resources. Approximately 1,800 of the world’s 10,000 bird species are long-distance migrants.
What is bird migration?
Bird migration is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. Migration is driven primarily by availability of food. Birds migrate to move from areas of low or decreasing resources to areas of high or increasing resources.
Why do birds migrate?
There are several key reasons birds migrate:
- Food availability – Birds migrate to find more abundant or nutritious food. Many birds migrate from northern breeding areas in summer to more southern wintering grounds with more available food in winter.
- Weather and climate – Birds migrate to avoid harsh winters, freezing temperatures, or drought.
- Breeding habitat – Some birds migrate to take advantage of different nesting and breeding habitats.
- Day length – Increasing day length in spring triggers migratory instincts to head north to breeding grounds.
- Interspecific competition – Migration reduces competition for food and nesting sites.
- Predation avoidance – Migrating reduces risk from predators at different times of year.
The primary driver for most bird migrations is food availability changes between seasons. But factors like weather, breeding habitat, day length cues, competition, and predation risk all help shape migratory behaviors.
How far do birds migrate?
Birds migration distances vary greatly. Some migrations cover only a few hundred miles, while the longest migrations may span tens of thousands of miles.
Short-distance migrants
Some species are short-distance migrants, traveling a few hundred to a few thousand miles between breeding and wintering areas. Examples include:
- American robin – Migrates from Canada/Alaska to U.S.
- Black-capped chickadee – Migrates across northeastern U.S. and Canada.
- Cedar waxwing – Migrates from across southern Canada to U.S.
These short-hop migrants move mainly to avoid harsh winters while still finding adequate food.
Medium-distance migrants
Medium-distance migrants may journey 1,000 to 3,500 miles between seasonal ranges. Examples include:
- Snow goose – Migrates 2,500 miles from Alaska and Canada to California and Mexico.
- Osprey – Migrates 1,500 miles from Canada to southern U.S. coasts.
- Bobolink – Migrates up to 3,000 miles between Canada/U.S. to South America.
These medium-distance migrations take the birds far enough south to escape freezing winter weather and find more abundant food supplies.
Long-distance migrants
The longest migrations see birds travel from Arctic and subarctic breeding zones to southern hemisphere wintering areas. Some extreme examples include:
- Arctic tern – Round-trip migration up to 50,000 miles between Arctic breeding grounds and Antarctic region.
- Bar-tailed godwit – Nonstop migration of 7,000 miles over 9 days from Alaska to New Zealand.
- Great snipe – 11,000 mile migration from Scandinavia and Siberia to Africa and South America.
- Swainson’s hawk – Up to 15,000 miles between Canada/U.S. to South America.
These marathon migrations minimize competition, escape freezing winters, and exploit southern food supplies.
What are the stages of bird migration?
Bird migration occurs in a seasonal cycle with several phases:
Preparation
Before migrating, birds enter a period of hyperphagia to build fat reserves to fuel migration. Changes in hormones like corticosterone help prepare for migration. Migratory restlessness and orientation behaviors develop.
Departure
To start migration, birds wait for optimal weather conditions before taking off. Nocturnal migrants typically leave in late afternoon or early evening. Diurnal migrants often leave early morning.
Travel
Birds make use of thermals, ridge lifts, and other wind patterns to aid migration. Most songbirds migrate at night. Larger birds migrate during the day. To cross barriers like mountains, deserts, or oceans, birds concentrate along flyways.
Stopover
After migrating a leg of a few hundred miles, birds stop to rest and feed for a period ranging from a few days to two weeks. This refuels fat stores for the next migratory leg.
Arrival
At breeding grounds, competition for nest sites and mates occurs. After breeding, the migration cycle reverses back to wintering areas.
What are the flyways used in bird migration?
Birds largely follow established flyways during migration. Major flyways in North America:
- Atlantic Flyway – Along eastern seaboard of U.S. and Canada
- Mississippi Flyway – Follows Mississippi River valley corridor
- Central Flyway – Between Rocky Mountains and Mississippi River
- Pacific Flyway – Along western coast of North America
Atlantic Flyway
The Atlantic Flyway follows the Atlantic coastline and Appalachian Mountains. Many eastern bird species follow this migratory path, including tundra swans, American woodcocks, and bobolinks.
Mississippi Flyway
The Mississippi Flyway overlaps the Mississippi River and its major tributaries. It’s a major migration route for waterfowl and shorebirds, including mallards, teal, and plovers.
Central Flyway
The Central Flyway spans the Great Plains between the Rocky Mountains and the Mississippi River. Birds like sandhill cranes, whooping cranes, and pelicans use this flyway.
Pacific Flyway
The Pacific Flyway covers migration routes along the western coast of North America. Many western species use it, including northern pintails, American wigeons, and white-crowned sparrows.
What navigation skills do birds use in migration?
Birds use incredible navigational skills to complete their marathon migrations. These include:
Compass orientation
Birds can sense the Earth’s magnetic field to help determine direction during migrations. One theory is birds have deposits of magnetite in their beaks and brains.
Sun compass
On sunny days, birds determine direction from the sun’s position. Birds compensate for the sun’s movement by recalibrating their internal compass.
Star and polarized light navigation
On clear nights, birds use the stars to orient themselves. Birds like European robins use polarized light patterns for orientation.
Landscape recognition
Birds utilize recognizable landscape features and landmarks as navigational cues, especially near breeding and wintering grounds.
Smell-based navigation
There is evidence that seabirds like shearwaters and petrels use their acute sense of smell to locate breeding islands.
Geomagnetic imprinting
In some species like salmon and sea turtles, juveniles imprint on local geomagnetic fields to later help guide return migration to natal breeding sites. Some birds may use similar magnetic imprinting.
What are the threats and challenges of bird migration?
Migration is an impressive feat, but also exposes birds to many threats and challenges:
Predation
Migrating birds are vulnerable to raptors and other predators during travel. One study found 14% of migrating songbirds were killed by predators on migration.
Starvation and exhaustion
Birds can starve or become critically exhausted if they cannot refuel adequately at stopovers along migration routes. Young birds with no migratory experience are especially at risk.
Severe weather
Storms, high winds, fog, and other weather can cause disorientation and blow birds off course. In spring weather mismatches, birds arrive before food is available.
Habitat loss
Wetland and forest loss along migratory flyways removes crucial stopover habitat. Fragmentation from roads, cities, and agriculture also degrades habitats.
Collisions
Collisions with buildings, communications towers, wind turbines, and other structures are a major hazard. It’s estimated 365 million to 1 billion birds die from building collisions annually in the U.S. alone.
Hunting
Some species are threatened by legal and illegal hunting along migration routes. For example, 5 million migratory birds are hunted legally each year in the Mediterranean region.
| Migration Challenge | Threats |
|---|---|
| Predation | Raptors, falcons, other predators |
| Starvation/exhaustion | Inadequate fat reserves and food refueling |
| Severe weather | Storms, high winds, fog, precipitation |
| Habitat loss | Wetland drainage, deforestation, fragmentation |
| Collisions | Buildings, towers, turbines, aircraft |
| Hunting | Legal and illegal harvest |
How does climate change affect bird migration?
Climate change poses many threats to migratory birds:
Timing mismatches
With warmer springs, birds are migrating earlier. This can lead to mismatches with peak food abundance, with serious impacts on reproductive success.
Range shifts
Rising temperatures may shift northern breeding ranges north. But southern wintering grounds may be unchanged, creating a migratory “funnel”.
Habitat shifts
Climate zones shift poleward 10 miles per decade on average. Habitats will change faster than some birds can migrate and adapt.
Extreme weather
Severe storms can blow migrating birds off course. Prolonged drought can degrade wetland stopover habitat.
Sea level rise
Rising seas will shrink and eliminate some coastal migratory stopover habitats. Low-lying breeding and wintering grounds will also be lost.
Reduced migration
Some birds are reducing migration distances in response to warmer winters and changes in food abundance. But shorter migrations may have downsides.
What can be done to protect migratory birds?
Several conservation strategies can help protect migratory birds:
Habitat conservation
Protect and restore wetlands and forests along migratory flyways to preserve essential stopover habitats. Promote bird-friendly agriculture and forestry.
Reduce collisions
Retrofit buildings and towers with anti-collision measures. Curtail building lighting during migration. Site turbines carefully.
Pollution reduction
Reduce pesticides and other chemicals that accumulate in food chains and poison birds. Regulate emissions that cause acid rain threatening breeding areas.
Hunting limits
Set science-based hunting regulations and bag limits. Crack down on illegal harvest of threatened migratory species.
International cooperation
Many migratory birds cross national borders and span continents. International treaties like the Migratory Bird Act establish shared conservation frameworks.
Education campaigns
Promote public awareness and appreciation for the wonders of bird migration to build support for conservation funding and policies.
Conclusion
Bird migration is a complex phenomenon driven mainly by seasonal food availability changes. Birds utilize sophisticated navigation skills to complete migrations up to tens of thousands of miles. But migration exposes birds to numerous threats like predation, habitat loss, and climate change. Protecting migratory birds requires habitat conservation, reducing human hazards on migration routes, and international cooperation to address long-distance migrations. With growing threats, understanding migration is key to effective conservation efforts to protect migrating birds into the future.
