Why do birds migrate

Migration is one of the most stunning phenomena on Earth. It means seasonal journey that birds undertake each year, moving between fixed breeding grounds and wintering sites. The scale and difficulty of this journey are beyond imagination. Some migratory birds travel only several kilometers, others must cross hundreds or even thousands of kilometers, sometimes flying over vast oceans or other challenging barriers.So, why do birds must to accomplish these impossible missions?

The main reason birds migrate is to access resources, particularly food, and to obtain suitable environments for breeding and overwintering.

Food is the most direct cause. Many birds feed on insects, aquatic life, plant seeds, or fruits. When winter arrives, temperatures in temperate and high-latitude regions drop below freezing, causing food resources decline or even vanish entirely. Migrating to lower latitudes means moving to an area where food remains abundant.

Breeding is also an important factor. During the breeding season, having better conditions to produce and raise more, stronger offspring increases the population’s chances of thriving.In the high-latitude summer, longer daylight hours provide parents more time to forage. Additionally, the distinct four-season climate in these regions leads to concentrated flowering, fruiting, and insect reproduction in spring and summer. This creates a plentiful food supply for parents feeding their youngs.

For other species, migration is an instinct written into their DNA. Initially, migration may have been a choice made by some individuals to secure resources. Over generations, established migration routes became a shared memory for the entire species.

For example, scientists have studied peregrine falcons. Through tracking and genetic analysis, scientists have found that the varying migration distances of them were shaped by historical climates, and completing long-distance journeys may rely on a strong memory-related gene.

Some birds, however, choose to stay in the "south."Bird migration does not follow a single pattern. In fact, it can be divided into three types: complete migration, partial migration, and full residency. Complete migration means the entire population migrates every year. In partial migration, some individuals in a population migrate while others stay in place. Full residency means the entire population stays in one area year-round.

Let’s take the Eurasian Blackbird (Turdus merula) as an example. Its different populations show a variety of migratory behaviors. Populations in Northern Europe and northern Russia are typical migratory birds. They fly south to the Mediterranean coast or North Africa for the winter. In contrast, many populations in Western Europe, Central Europe, and central-southern China are resident birds. They live in their breeding areas all year round. In parts of Central Europe, resident and migratory blackbirds often live side by side. This is an example of the partial migration mentioned earlier.

Researchers have not yet identified a single deciding factor. Evidence suggests that daylight, food availability, hormones, and other conditions may all influence when birds begin their migration.

Birds rely on changes in day length to sense the arrival of spring. As days grow longer, special light-sensitive cells in their brains trigger preparations: growing new feathers, eating more to store energy, strengthening flight muscles, and getting ready to breed.

For some birds, temperature also plays a role. If climate change causes warmer springs to arrive earlier, migrating sooner may offer an advantage. Species such as Red-winged Blackbirds, Common Grackles, Eastern Phoebes, and Tree Swallows have been observed migrating earlier in warmer springs compared to others of their kind.

Other birds are driven by internal hormonal or cyclical rhythms, leading them to follow a strict migration schedule. Even under constant laboratory conditions, with fixed light and temperature, these migratory birds still show periodic restlessness and attempt to take flight. This indicates that birds possess an innate internal annual rhythm that drives them to prepare 

Migratory birds can be divided into two main types based on what prompts their journey: obligate migrants and facultative migrants.For obligate migrants, migration is driven by instinct. They depart and arrive around the same time each year. While severe weather may delay travel, these birds generally adhere to their internal schedule. Even if spring arrives early in an area, that does not mean obligate migrants will arrive sooner.Facultative migrants, however, have more flexibility. Their migration is influenced by physical changes in their environment. In autumn and winter, they may migrate in search of better food supplies. In spring, rising temperatures can trigger their journey.

Indeed, how birds navigate is a complex question. Researchers have found that birds use many different methods to find their way, varying by species. Birds mainly rely on natural cues from their environment to determine direction. They can navigate using the sun, the stars, the Earth's magnetic field, visible landmarks, and even smells and sounds to stay on course and correct their route.

A bird's ability to navigate is partly innate and partly learned. They combine multiple clues and cross-check them against each other. They also switch navigation methods depending on conditions. For example, on cloudy days a bird might use the magnetic field instead of the sun, and at night it might use the stars instead. Young birds may learn the migration route by following adults. In some species, such as geese and cranes, family groups migrate together, led by parents.

For birdwatchers, witnessing the waves of migrating birds can be a breathtaking experience. However, as explained earlier, bird migration is a complex behavior influenced by many factors. Even within the same species, migration patterns can vary widely.Here are some useful tools to help you better prepare for your next birdwatching trip.

https://explorer.audubon.org/home?sidebar=expand

This is an online platform for bird migration data. It integrates real-time tracking information from research institutions around the world to visually display the migration routes of different bird species. You can explore which birds appear in specific areas and see their migration paths, along with the actual survival threats they face during their journey. You can also select species you are particularly interested in and watch an animated display of their annual migration routes.

https://birdcast.info/migration-tools/

The BirdCast platform develops and maintains a suite of tools for predicting and monitoring bird migration. Below are all four tools it offers.

https://birdcast.info/migration-tools/migration-forecast-maps/

This tool shows the predicted intensity of nocturnal migration for the three hours after local sunset. It updates every six hours.

https://birdcast.info/migration-tools/live-migration-maps/

This map helps you view near-real-time analysis of actual nocturnal bird migration intensity detected by U.S. weather surveillance radar networks from sunset to sunrise.

https://birdcast.info/migration-tools/local-migration-alerts/

Use this tool to determine if birds are migrating over your city tonight. It classifies bird migration in a specific area as low, medium, or high density, providing timely information about migration peaks. These peak nights occur only a few times each migration season.

https://birdcast.info/migration-tools/migration-dashboard/

Explore nocturnal migration data for your location with this dashboard. It includes estimates of the total number of migrating birds, along with their direction, speed, and altitude. It also provides historical records (2013–2021) and data on past nights of migration activity.The live data feed runs from March 1 to June 15 during spring migration and from August 1 to November 15 during fall migration.

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