The seasonal movement of birds from one region to another is called migration. Migration allows birds to find more favorable conditions for feeding, breeding, and raising their young. Some birds may migrate relatively short distances, while others undertake remarkably long journeys of thousands of miles. Migration is an amazing feat of endurance that birds are adapted for. Understanding bird migration helps explain why certain species occur in an area only at certain times of the year.
What causes birds to migrate?
Birds migrate to take advantage of favorable conditions during different seasons. The key factors motivating migration are food availability, climate, and reproduction. Birds migrate from areas of low or declining resources to areas of high or increasing resources. The availability of food is especially important. For example, many species of warblers summer in North America where there are abundant insects for them to eat. But as winter approaches and insect numbers decline, the warblers migrate to Latin America where insects remain plentiful all year. Climate is another factor. Birds leave areas that become too cold in winter for regions that remain mild. Migration allows birds to nest in summer areas with the abundant food needed to raise young. Leaving wintering grounds allows those areas to be available for a different set of bird species better adapted for cold conditions.
What are the benefits of migration?
The main benefits of migration include:
– Access to more abundant or stable food supplies.
– Moving to areas with favorable climates.
– Taking advantage of longer daylight hours to breed during the summer.
– Avoiding extreme weather and scarcity of food in winter.
– Reduced competition for resources that occur when different species occupy the same areas during summer versus winter.
Migration allows birds to opportunistically take advantage of seasonal conditions and resources. Birds can nest and raise young when food is plentiful, then move to areas where food remains available when breeding areas become inhospitable. Over evolutionary time, migrating between different areas during summer and winter has allowed diverse species to flourish around the world.
When does bird migration occur?
Spring migration
In spring, birds migrate from their wintering grounds to their summer breeding areas. In North America, most spring migration takes place between March and May. The exact timing depends on the species and local conditions. Spring migration usually starts as winter ends and food becomes scarce. It also coincides with improving weather and longer daylight hours useful for breeding. Some short-distance migrants start moving early while long-distance migrants wait longer before migrating. Males often migrate before females so they can establish territories before the females arrive.
Fall migration
Fall migration occurs between late August and November in North America as birds move from their summer breeding areas to their wintering grounds. The timing depends on the species. Short-distance migrants often start migrating earlier while long-distance migrants wait longer before migrating. Fall migration typically begins after the breeding season ends and food starts becoming scarce. The amount of daylight also influences migration timing, with less daylight prompting earlier migration. Young birds produced during the summer often migrate separately from adults. Weather such as cold snaps and storms may lead birds to initiate migration earlier as well.
What are the different types of bird migration?
There are several different types of bird migration strategies:
Longitude migration
Birds that migrate relatively straight north and south are longitude migrants. Many North American songbirds follow routes between North and South America. Species such as warblers, tanagers, orioles, and flycatchers are examples of longitude migrants. They may travel several hundred to a few thousand miles between their breeding and wintering grounds.
Latitude migration
Latitude migrants travel primarily east and west rather than north and south. Examples are birds moving between Europe and Africa or Asia and Australia. Latitude migration often involves somewhat shorter distances than longitude migration. Species such as wheatears, barn swallows, and common cranes are latitude migrants.
Elevational migration
Some birds migrate up and down mountains and hillsides rather than long distances. Movement is typically between higher elevations in spring and summer for breeding and lower elevations in fall and winter. In the Himalayas, pheasants and cranes elevational migrants. Hummingbirds in the Andes also migrate uphill and downhill between breeding and nonbreeding areas.
Irruptive migration
Irruptive migrants are species that normally migrate shorter distances but make occasional longer migrations when food supplies crash. Examples include crossbills, snowy owls, Bohemian waxwings, and grosbeaks. By migrating further afield, these species can find abundant food when it becomes scarce locally. Their irregular migration is tied to fluctuating food supplies.
Partial migration
In partial migration, some individuals or populations migrate while others remain in the same area year-round. For example, American robins may migrate from Canada to the southern US or Mexico while others remain through winter in milder northern climates. Ducks and geese also show partial migration, with northernmost breeding populations migrating the farthest while southern breeders migrate shorter distances or not at all.
Nomadic migration
Nomadic species wander irregularly, tracking unpredictable food and water supplies over a large territory. They do not follow predictable seasonal migration patterns. Examples are larks, swifts, and birds of prey like peregrine falcons that may wander widely based on prey availability. Their nomadic movements allow flexibility to take advantage of shifting resources.
What are the longest bird migrations?
Some of the most remarkable marathon migrations include:
Arctic tern
– Breeds: Arctic region
– Winters: Antarctic region
– One way distance: 22,000 – 25,000 miles
– Total roundtrip: 44,000 – 50,000 miles
Arctic terns hold the record for the longest migration, zigzagging between the Arctic and Antarctic each year. Their route includes oceanic flights and coastal legs that follow cold waters teeming with fish and krill.
Bar-tailed godwit
– Breeds: Alaska and Siberia
– Winters: New Zealand and Australia
– One way distance: ~7,000 miles
– Total roundtrip: ~14,000 miles
Godwits make nonstop transoceanic flights travelling from Alaska to New Zealand in just 6-8 days without food or water. Their long beaks allow them to probe deeply in mudflats to fuel up before migrating.
Ruby-throated hummingbird
– Breeds: Eastern North America
– Winters: Central America
– One way distance: ~500 miles across Gulf of Mexico
– Total roundtrip: up to 3,000 miles
Though small, hummingbirds migrate remarkably long distances, with many crossing the Gulf of Mexico nonstop in 20-28 hours. Their high metabolism powers their stamina and endurance.
Great snipe
– Breeds: Scandinavia and Russia
– Winters: Africa
– One way distance: 4,200 miles
– Total roundtrip: 8,400 miles
Great snipes fly from Scandinavia to Africa without stopping, averaging up to 55 mph on their 4,200 mile journey over the Mediterranean Sea and Sahara Desert. They can lose half their body weight during the trip.
Sooty shearwater
– Breeds: New Zealand and southern oceans
– Winters: North Pacific off Japan and California
– One way distance: 5,000 – 9,000 miles
– Total roundtrip: 14,000+ miles
Sooty shearwaters make figure eight migrations each year through Pacific and Atlantic Oceans. They can cover over 40,000 miles annually following a zigzagging pattern chasing resources.
How do birds navigate during migration?
Birds use a combination of innate abilities and learned cues to navigate during migration.
Instinct and genetics
Birds have innate guidance systems tuned by evolution that provide directional and spatial information. Genetics influence migration timing and routes. Young birds on their first migration can orient towards the right direction despite never migrating before.
Compass senses
Birds sense the Earth’s magnetic field for directional information. They also use the sun as a compass reference during the day. At night, birds orient using star patterns.
Landmark recognition
Birds use recognizable features and landmarks to navigate during migration. Individuals learn key navigation points during their first migration and use these for subsequent journeys. Landmarks include coastlines, mountain ranges, rivers, and regional habitats.
Celestial cues
The positions of the sun, stars, and patterns of polarized light provide navigational information for migrating birds. These celestial cues help birds maintain their compass direction. The duration of light also influences migration timing.
Weather monitoring
Birds can detect air pressure changes and wind directions to assist navigation. Headwinds may prompt birds to stopover to wait for better winds. Birds can fly below bad weather and ride favorable tailwinds to aid migration.
Sense of smell
There is evidence some birds use their sense of smell to help locate stopover sites, particularly at sea and on uniform landscapes like deserts. Birds may smell prey items, plant aromas, or other scents to pinpoint landing areas.
What are the hazards and threats migrating birds face?
Migrating birds face many challenges and threats on their strenuous journeys:
Starvation
Building sufficient fat reserves to fuel migration takes substantial food resources. Birds can starve if they cannot find enough food at stopover sites. Inclement weather can prevent feeding and fat replenishment.
Predators
Birds are vulnerable to predators, particularly at stopovers when fatigued. Falcons, hawks, eagles, and owls prey on migrating songbirds.
Severe weather
Storms, high winds, fog, and precipitation can blow migrating birds off course or force them to land. Exhausted birds face exposure risks.
Human structures
Windows, power lines, wind turbines, towers, and other structures kill huge numbers of migrating birds that collide with them. Light pollution disorients night migrants.
Habitat loss
Wetland drainage and development along migratory routes removes crucial stopover habitats for refueling and resting.
Climate change
Changing temperatures, weather patterns, vegetation, and food webs disrupt evolved migration timing tuned to historic conditions. Mismatch results.
Hunting
Legal and illegal shooting takes a toll on some migrating waterfowl, doves, and other game species. For songbirds, hunting pressure occurs more on wintering grounds.
Why is bird migration important?
Bird migration is critical for birds and ecosystems for several reasons:
Seasonal habitat usage
Migration allows different bird species to use the same areas during different parts of the year, increasing overall biodiversity.
Food availability
By moving, birds access peaks of food abundance in different regions, supporting their breeding, winter survival, and migration energy needs.
Optimal breeding conditions
Migration allows birds to nest in seasonal locations with ideal conditions for raising young. The long daylight hours farther north in summer provide more feeding time.
Winter survival
Migrating saves birds from starvation or cold stress in harsh northern winters. Birds move to warmer areas with adequate food.
Ecosystem links
Migrating birds transport nutrients between ecosystems and are an important food source for predators along their routes. Their abundance fluctuates seasonally.
Evolutionary adaptation
The ability to migrate has allowed bird species to thrive across large ranges and fill diverse niches through seasonal movement.
Conclusion
Bird migration is a captivating phenomenon. Birds undertake arduous long-distance journeys in spring and fall between breeding and wintering grounds. Their marathon flights take advantage of seasonal peaks in food availability, climate conditions, and daylight favorable for nesting. Navigation involves a combination of innate abilities and learned cues. Many hazards including bad weather, habitat loss, predators, and human structures threaten migrating birds. Protecting stopover habitats and migration flyways is crucial. Bird migration is an important ecological adaptation that adds beauty, biodiversity, and ecosystem functionality around the globe through the seasonal redistribution of billions of birds. The phenomenon connects ecosystems, seeds dispersal and nutrient transport, and provides abundant opportunities for humans to study and observe some of the most incredible feats of endurance in the animal kingdom. Bird migration is a unique and inspiring example of the seasonal rhythms of nature.
| Migration Type | Definition | Examples |
|---|---|---|
| Longitude | North-south migration between breeding and wintering grounds | Warblers, orioles, flycatchers |
| Latitude | East-west migration between continents | Barn swallows, common cranes |
| Elevational | Up and downhill migration in mountains | Pheasants, hummingbirds |
| Irruptive | Irregular long-distance migration when food is scarce | Snowy owls, Bohemian waxwings |
| Partial | Only some individuals or populations migrate | American robins, ducks |
| Nomadic | Wandering movement following resources | Larks, swifts, peregrine falcons |
