Bird migration

Bird migration is a fascinating and important natural event. Many birds travel long distances between their breeding areas and their wintering grounds each year, usually moving between northern and southern regions. This journey happens twice a year and is full of challenges, as birds face dangers such as predators along the way.

One amazing bird, the Arctic tern, holds the record for the longest migration, traveling all the way from its Arctic breeding grounds to the Antarctic every year. Other birds, like albatrosses, which are a type of tubenoses, fly in circles over the southern oceans. Some birds, such as Manx shearwaters, travel an incredible 14,000 kilometers (8,700 miles) from their homes in the north to the Southern Ocean.

Many birds don’t travel such great distances. Shorter trips, like moving up and down mountains, are also common. For example, birds in the Andes and Himalayas change their homes with the seasons.

Birds know when and where to go mostly because of changes in the length of daylight. They use many tools to find their way, such as the position of the Sun and stars, the Earth’s magnetic field, and special maps they keep in their minds.

Historical views

People have noticed bird migration for thousands of years. In the Pacific, ancient traditions tell stories of birds helping people find land. For example, in Samoan tales, a bird named Tuli guided the way.

Ancient Greeks also wrote about birds coming and going with the seasons. Aristotle observed that cranes traveled from places near the Nile to other lands. For a long time, some people believed birds like swallows hibernated instead of migrating. It wasn’t until the late 1700s that scientists widely accepted that birds actually migrate. One famous bird book from 1797 described seeing swallows flying north, supporting the idea that they leave when food is scarce. In 1822, a white stork was found in Mecklenburg with an arrow from central Africa, showing that some birds travel very long distances.

General patterns

Migration is the regular seasonal movement of birds between breeding and non-breeding areas, usually north and south. Many birds travel long distances along specific routes called flyways. In spring, they fly north to breed, and in autumn, they return south to warmer regions. This movement helps birds find food and better breeding conditions, even though it is tiring and dangerous.

Some birds migrate in flocks, which helps them save energy. For example, geese flying in a V formation use less energy than flying alone. Not all birds in a species migrate; some stay in one place year-round. Migration routes can be influenced by geography, weather, and even learned from other birds.

Nocturnal migratory behaviour

Many birds migrate at night, making short calls to stay together and avoid collisions. Scientists can track these night-time journeys using weather radar to see where birds are flying and how many there are. These birds rest and eat when the sun rises before continuing their trip.

Flying at night helps birds stay safe from predators and stay cool, though it means they miss out on sleep. They can adjust their sleep to make up for this lost rest.

Long-distance migration

The typical image of migration is of northern land birds, such as swallows and birds of prey, making long flights to the tropics. However, many Holarctic wildfowl and finch (Fringillidae) species winters in the North Temperate Zone, in regions with milder winters than their summer breeding grounds. For example, the pink-footed goose migrates from Iceland to Britain and neighbouring countries, whilst the dark-eyed junco migrates from subarctic and arctic climates to the contiguous United States and the American goldfinch from taiga to wintering grounds extending from the American South northwestward to Western Oregon.

Migration routes and wintering grounds are both genetically and traditionally determined depending on the social system of the species. In long-lived, social species such as white storks, flocks are often led by the oldest members and young storks learn the route on their first journey. In short-lived species that migrate alone, such as the Eurasian blackcap or the yellow-billed cuckoo, first-year migrants follow a genetically determined route.

Many migration routes of long-distance migratory birds are circuitous due to evolutionary history. For most land birds, such barriers could consist of large water bodies or high mountain ranges, a lack of stopover or feeding sites, or a lack of thermal columns (important for broad-winged birds). Conversely, in water-birds, large areas of land without wetlands offering suitable feeding sites may present a barrier, and detours avoiding such barriers are observed. For example, brent geese migrating between the Taymyr Peninsula and the Wadden Sea travel via low-lying coastal feeding-areas on the White Sea and the Baltic Sea rather than directly across the Arctic Ocean and the Scandinavian mainland.

Great snipes make non-stop flights of 4,000–7,000 km, lasting 60–90 h, during which they change their average cruising heights from 2,000 m (above sea level) at night to around 4,000 m during daytime.

A similar situation occurs with waders (called shorebirds in North America). Many species, such as dunlin and western sandpiper, undertake long movements from their Arctic breeding grounds to warmer locations in the same hemisphere, but others such as semipalmated sandpiper travel longer distances to the tropics in the Southern Hemisphere.

For some species of waders, migration success depends on the availability of certain key food resources at stopover points along the migration route. This gives the migrants an opportunity to refuel for the next leg of the voyage. Some examples of important stopover locations are the Bay of Fundy and Delaware Bay.

Some bar-tailed godwits have the longest known non-stop flight of any migrant, flying 11,000 km from Alaska to their New Zealand non-breeding areas. Prior to migration, 55 percent of their bodyweight is stored as fat to fuel this uninterrupted journey.

Seabird migration is similar in pattern to those of the waders and waterfowl. Some, such as the black guillemot and some gulls, are quite sedentary; others, such as most terns and auks breeding in the temperate northern hemisphere, move varying distances south in the northern winter. The Arctic tern has the longest-distance migration of any bird, and sees more daylight than any other, moving from its Arctic breeding grounds to the Antarctic non-breeding areas. Many tubenosed birds breed in the southern hemisphere and migrate north in the southern winter.

The most pelagic species, mainly in the 'tubenose' order Procellariiformes, are great wanderers, and the albatrosses of the southern oceans may circle the globe as they ride the "Roaring Forties" outside the breeding season. The tubenoses spread widely over large areas of open ocean, but congregate when food becomes available. Many are among the longest-distance migrants; sooty shearwaters nesting on the Falkland Islands migrate 14,000 km between the breeding colony and the North Atlantic Ocean off Norway. Some Manx shearwaters do this same journey in reverse. As they are long-lived birds, they may cover enormous distances during their lives.

Some large broad-winged birds rely on thermal columns of rising hot air to enable them to soar. These include many birds of prey such as vultures, eagles, and buzzards, but also storks. These birds migrate in the daytime. Migratory species in these groups have great difficulty crossing large bodies of water, since thermals only form over land, and these birds cannot maintain active flight for long distances. Mediterranean and other seas present a major obstacle to soaring birds, which must cross at the narrowest points. Massive numbers of large raptors and storks pass through areas such as the Strait of Messina, Gibraltar, Falsterbo, and the Bosphorus at migration times.

Short-distance and altitudinal migration

Main article: Altitudinal migration

Many birds that travel short distances move based on local weather rather than changes in day length. For example, birds living in mountains, like the wallcreeper and white-throated dipper, move to lower areas when it gets too cold. Other birds, such as the merlin and Eurasian skylark, travel to coastal areas or further south.

Some birds, like the waxwings, move because winter weather affects their food supply. In tropical areas, birds may move up or down mountains to find more of their favorite foods, like fruits. This type of movement happens in many mountain ranges around the world, including the Himalayas and the Andes.

Irruptions and dispersal

Sometimes, when there is a good breeding season followed by a lack of food the next year, many birds move far beyond their usual areas. This is called an irruption. For example, Bohemian waxwings have had five major arrivals in Britain during the nineteenth century, but 18 between 1937 and 2000. Red crossbills have also had widespread invasions across England in years such as 1251, 1593, 1757, and 1791.

Bird migration happens mostly in the Northern Hemisphere because the land there often has less food in winter, so birds fly south to find more. However, many seabirds from the Southern Hemisphere also migrate because there is a lot of ocean and many islands where they can nest.

Physiology and control

The timing of bird migration is mainly controlled by changes in day length, which trigger hormonal changes in the birds. Before migration, birds often become more active and build up extra fat to prepare for their journey. Even birds kept in cages show signs of wanting to migrate, showing that this behavior is programmed within them.

Birds use many tools to navigate during migration. They can use the sun, Earth’s magnetic field, landmarks, and even smells to find their way. Young birds may follow the Earth’s magnetic field on their first journey, while older birds learn from experience and improve their navigation skills over time. Sometimes birds can get lost and end up far from where they intended to go.

Adaptations

Birds change their bodies to prepare for migration. They store energy by building up fat and adjust their sleep patterns, especially those that fly at night. Their feathers also wear out and need to be replaced, which happens either before they leave for winter or before they return to breed.

Migration can also change how birds behave. For example, many birds fly in groups to save energy and stay safer from predators.

Evolutionary and ecological factors

Bird migration is a flexible behavior that has developed separately in many bird groups. Scientists believe that while genes play a role in migration, some birds can develop migratory behaviors without genetic changes. This helps explain how migration appeared quickly after the last big ice age.

Climate change is affecting when birds migrate. Birds usually move to find food and breeding grounds, but changes in weather patterns can cause timing mismatches. For example, if plants grow earlier but birds still migrate on their old schedule, they might miss the best food sources. These timing shifts are already being observed in many bird species across large areas. Some birds with flexible diets may handle these changes better than others.

Main article: Climate change and birds

Ecological effects

Bird migration helps move other species, including tiny creatures like ticks and lice that can carry germs. While some diseases, like avian influenza, can spread through birds, studies show that migration itself isn't a big risk — bringing in pet and domestic birds can be more dangerous. Migrating birds can also spread viruses such as the West Nile virus and help plants and tiny water creatures called plankton travel to new places.

During migration, some animals take advantage of the large groups of birds. For example, Greater noctule bats hunt birds that fly at night, and some birds of prey follow groups of shorebirds.

Study techniques

Scientists have studied bird migration in many interesting ways. One of the oldest methods is marking birds, like putting a small nick on a swan's beak. More recently, scientists have used radar and satellite tracking to follow birds as they fly. They also use special tools like the Emlen funnel to see how birds figure out their direction.

Other techniques include using stable isotopes from bird feathers to trace where birds go and listening to bird calls at night with microphones. These different methods help us learn more about how and when birds travel during migration.

Threats and conservation

Human activities pose many threats to migratory birds. Because birds travel long distances, they often cross many country borders, so protecting them needs cooperation between nations. Several treaties, like the Migratory Bird Treaty Act of 1918 in the US and the African-Eurasian Migratory Waterbird Agreement, help protect these birds.

During migration, birds can face dangers such as hunting, especially in places like Afghanistan and Central Asia, which affected Siberian cranes. Structures like power lines, wind farms, and oil rigs also pose risks. Other hazards include pollution, storms, and loss of habitats where birds rest and eat. Many important stopover sites have been lost due to farming, making it harder for birds to survive their journeys.

Conservation efforts are helping. In California, farmers flood their fields in winter to create temporary wetlands for birds to rest and eat. This helps birds like the White-fronted Goose and reduces the need for chemical fertilizers, benefiting both birds and farmers. However, too many birds in one place can harm water quality, so spreading out these habitats is important.