Bird migration

Bird migration is a fascinating and important natural event where many birds travel long distances between their breeding grounds and their wintering grounds each year. These journeys usually happen between northern and southern regions, helping birds find food and safe places to raise their young. Migration can be dangerous because birds face threats from predators and other risks along the way.

One of the most amazing migrators is the Arctic tern, which holds the record for the longest migration of any bird. Each year, it travels all the way from its breeding grounds in the Arctic to the Antarctic. Other birds, like albatrosses and Manx shearwaters, also fly incredible distances, sometimes circling the Earth or traveling thousands of kilometers between their homes.

Birds know when and where to migrate mostly by changes in the length of daylight. They use clues from the Sun, the stars, and the Earth's magnetic field to guide their way. These amazing journeys show how clever and strong birds are as they travel across the world every year.

Historical views

People have noticed bird migration for thousands of years. In the Pacific, traditional practices of the Micronesians and Polynesians show that they observed birds flying to find new lands. For example, in Samoan stories, a bird named Tuli helped find dry land.

Ancient writers like Aristotle and Pliny the Elder wrote about birds appearing and disappearing with the seasons. They sometimes thought birds hid or slept through winter, but later scientists learned that birds actually travel to warmer places. In the late 1700s, Thomas Bewick helped show that swallows leave when food is scarce. In 1822, a white stork found in Mecklenburg with an arrow from Africa gave clear proof that some birds fly very far.

General patterns

Migration is the regular seasonal movement of many bird species between breeding areas and areas where they spend the winter. Birds usually travel north in the spring to breed and return south in the autumn when the weather gets colder. This helps them find food and take advantage of longer days for raising their young.

Some birds travel very long distances along special paths called flyways. Many birds fly in groups, which helps them save energy. While most birds fly during migration, some, like penguins, swim to their destinations instead.

Nocturnal migratory behaviour

Many birds migrate at night, a behavior called "nocmig." During these night journeys, birds make short calls to stay in touch with their flock and avoid flying into each other. Scientists can watch these night-time migrations using weather radar to learn how many birds are moving and where they are going.

These night-migrating birds usually land in the morning, rest and eat for a few days, and then continue their journey. They are called passage migrants in the areas they pass through. Flying at night helps birds avoid predators and staying too hot, though it does mean they lose some sleep.

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 (Ciconia ciconia), 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 Sylvia atricapilla or the yellow-billed cuckoo Coccyzus americanus, first-year migrants follow a genetically determined route that is alterable with selective breeding.

Short-distance and altitudinal migration

Main article: Altitudinal migration

Many birds do not travel very far when they move between breeding and winter areas. Instead, they might move up or down mountains to find better weather and food. For example, birds like the wallcreeper and white-throated dipper live in cold mountain areas and move to lower elevations when it gets too cold.

Some birds in tropical areas also move up and down mountains to find their favorite foods, like certain fruits. In places like the Himalayas and the Andes, birds such as the Dusky grouse make small moves to find better winter spots. Other birds, especially in dry areas like southern Australia, travel around following water and food whenever they appear.

Irruptions and dispersal

Sometimes, when birds have a great breeding season but then face a shortage of food the next year, many of them travel far beyond their usual areas. This is called an irruption. For example, Bohemian waxwings have had many such arrivals in places like Britain. Another example is red crossbills, which have also been seen far from their normal homes in various years.

Most bird migration happens in the Northern Hemisphere. This is because the land there often has less food in winter, so birds travel south to find more. However, some seabirds from the Southern Hemisphere also migrate, especially because there is more ocean and islands there for them to live on.

Physiology and control

The timing and control of bird migration are mainly driven by genetics and are present even in birds that don’t migrate. Birds use changes in daylight as a key signal for when to start migrating, which also triggers changes like increased activity and fat storage.

Birds navigate during migration using a mix of senses. They can use the sun, magnetic fields, visual landmarks, and even smells to find their way. Young birds follow innate directions based on the Earth’s magnetic field, while older birds learn from experience and improve their navigation over time. Some birds can even sense the Earth’s magnetic field directly, helping them stay on course.

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, usually once a year, though sometimes twice. Some birds replace their feathers before going to their winter homes, while others do it before returning to where they breed.

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

Evolutionary and ecological factors

Bird migration is a flexible behavior that has developed in many bird families over time. While genetics play a role in enabling birds to migrate, some believe that migratory behavior can appear in birds that normally stay in one place because the basic ability to migrate exists in almost all birds. This might explain why migration appeared quickly after the last big ice age.

Climate change is also affecting when and how birds migrate. As temperatures change, birds may start their journeys earlier or later than before. This can sometimes cause problems because the food and conditions they rely on might not be available when they arrive. Some birds with flexible diets may handle these changes better than others.

Ecological effects

Bird migration helps move other species, including tiny pests like ticks and lice, which can carry germs. Some diseases, such as avian influenza, have been studied to see if birds spread them during migration, but it turns out that bringing in pet and domestic birds poses a bigger risk. Migrating birds can also spread viruses like 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 many birds gathered together. For example, greater noctule bats hunt nighttime-flying birds, and some birds of prey focus on catching migrating shorebirds.

Study techniques

Bird migration has been studied using many different methods. One of the oldest ways is marking birds, such as nicking a swan's beak. Modern scientists use radar and satellite tracking to follow birds' paths. They also use special tools like the Emlen funnel to see how birds find their direction.

Scientists can learn where birds travel by studying stable isotopes in their feathers. They also listen to bird calls at night to learn more about migration. These many techniques help us understand how and why birds migrate.

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. Important agreements like the Migratory Bird Treaty Act help safeguard these species.

During migration, birds gather in large groups, making them vulnerable. Some birds, like the passenger pigeon, have sadly disappeared. Hunting, power lines, and loss of habitats along their routes also danger their journeys. Conservation efforts are vital to help birds find safe places to rest and eat during their travels.