Bird migration

Bird migration is a wonderful natural event. Many birds travel far between their homes in the summer and their homes in the winter. They usually fly between northern and southern areas. This trip happens twice a year and has many challenges.

One amazing bird, the Arctic tern, travels the farthest. It flies all the way from its home in the Arctic to the Antarctic each 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 a very long distance of 14,000 kilometers (8,700 miles) from their homes in the north to the Southern Ocean.

Many birds do not travel such long 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, old stories tell of birds helping people find land. For example, in Samoan tales, a bird named Tuli showed the way.

Ancient Greeks also wrote about birds arriving and leaving with the seasons. Aristotle saw that cranes traveled from places near the Nile to other lands. For a long time, some people thought birds like swallows went to sleep instead of moving. It wasn’t until the late 1700s that scientists agreed that birds migrate. A famous bird book from 1797 told of seeing swallows flying north, proving they leave when food is hard to find. In 1822, a white stork was found in Mecklenburg with an arrow from central Africa, showing that some birds travel far.

General patterns

Migration is the regular seasonal movement of birds between places where they breed and places where they spend the rest of the year, usually north and south. Many birds travel long distances along special paths called flyways. In spring, they fly north to breed, and in autumn, they fly south to warmer areas. This helps birds find food and good places to breed, even though it is hard work.

Some birds migrate in groups, which helps them save energy. For example, geese flying in a V formation use less energy than flying alone. Not all birds in a group migrate; some stay in one place all year. Migration paths can be affected by the land, the weather, and what they learn from other birds.

Nocturnal migratory behaviour

Many birds fly south at night. They make short calls to stay together and avoid bumping into each other. Scientists use weather radar to watch where birds fly and how many are traveling. These birds rest and eat when the sun comes up before flying again.

Flying at night helps birds stay safe from animals that might want to eat them. It also keeps them cooler. But it means they do not get as much sleep. Birds can catch up on their rest later.

Long-distance migration

The typical image of migration is of northern land birds, such as swallows and birds of prey, making long flights to warmer places. Many Holarctic wildfowl and finch (Fringillidae) species spend winter in the North Temperate Zone, where winters are milder than where they breed in summer. For example, the pink-footed goose migrates from Iceland to Britain and nearby countries, while the dark-eyed junco migrates from very cold areas to the United States. The American goldfinch moves from forest areas to winter areas from the American South to Western Oregon.

Migration routes and winter areas are decided by both family traits and learning. In long-lived, social birds such as white storks, groups are led by older birds, and young storks learn the way during their first trip. In short-lived birds that travel alone, such as the Eurasian blackcap or the yellow-billed cuckoo, young birds follow a path set by their genes.

Many migration paths for long-distance birds are indirect because of where they can stop and rest. For land birds, big bodies of water or high mountains can block their way, as well as a lack of places to rest and eat, or a lack of thermal columns (important for birds with broad wings). For water-birds, large areas of land without wetlands can be a problem, and birds may take longer routes to avoid these areas. For example, brent geese traveling between the Taymyr Peninsula and the Wadden Sea fly over low coastal areas near the White Sea and the Baltic Sea instead of going directly over the Arctic Ocean and Scandinavian lands.

Great snipes can fly nonstop for 4,000–7,000 km, taking 60–90 hours. During these flights, they fly higher at night and lower during the day.

A similar pattern is seen in waders (called shorebirds in North America). Many species, such as dunlin and western sandpiper, travel from their Arctic breeding areas to warmer places in the same part of the world, while others such as semipalmated sandpiper fly to the tropics in the Southern Hemisphere.

For some wader species, successful migration depends on finding certain foods at stops along the way. These stops let the birds rest and get food for the next part of their journey. Important stopover places include the Bay of Fundy and Delaware Bay.

Some bar-tailed godwits have the longest known nonstop flight of any migratory bird, traveling 11,000 km from Alaska to New Zealand. Before they leave, they build up fat to give them energy for this long trip.

Seabird migration follows patterns similar to waders and waterfowl. Some, like the black guillemot and some gulls, move only short distances; others, such as most terns and auks that breed in the northern temperate areas, travel south in the northern winter. The Arctic tern has the longest migration of any bird, traveling from its Arctic breeding areas to the Antarctic to spend the winter. Many birds that nest in the southern hemisphere fly north during the southern winter.

The most ocean-going birds, mainly in the group Procellariiformes, travel widely. The albatrosses of the southern oceans can circle the globe during their non-breeding time. These birds spread out over large areas of open ocean but come together when food is available. Many are among the longest-distance travelers; sooty shearwaters that nest on the Falkland Islands migrate 14,000 km to the North Atlantic Ocean near Norway. Some Manx shearwaters make this trip in the opposite direction. Because they live for many years, they can travel huge distances in their lifetimes.

Some large birds with broad wings rely on rising warm air to glide. This includes many birds of prey such as vultures, eagles, and buzzards, as well as storks. These birds travel during the day. Birds in these groups have a hard time crossing big bodies of water because the rising warm air they need only forms over land, and they cannot fly actively for long distances. Mediterranean and other seas are major challenges for these gliding birds, which must cross at the narrowest points. Large numbers of raptors and storks pass through places such as the Strait of Messina, Gibraltar, Falsterbo, and the Bosphorus when they are migrating.

Short-distance and altitudinal migration

Main article: Altitudinal migration

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

Some birds, like the waxwings, move when winter weather changes their food. In tropical areas, birds may move up or down mountains to find more food, 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 birds have a good year for raising their young but then find less food the next year, many birds fly far beyond their usual areas. This is called an irruption. For example, Bohemian waxwings have visited Britain often, with many arrivals between 1937 and 2000. Red crossbills have also been seen widely across England in different years.

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. 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

Bird migration is mostly controlled by changes in the length of days. These changes cause birds to have hormonal shifts. Before they migrate, birds often become more active and gain extra fat to get ready for their trip. Even birds in cages show signs that they want to migrate, which shows this behavior is natural for 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 might follow the Earth’s magnetic field on their first journey, while older birds learn and get better at navigating over time. Sometimes birds can get lost and end up far from where they planned to go.

Adaptations

Birds change their bodies to get ready for migration. They build up fat to give them energy. Birds that fly at night change their sleep patterns. Their feathers get old and need to be replaced. This happens before they leave for winter or before they return to breed.

Migration can also change how birds act. Many birds fly together in groups. This helps them save energy and stay safer from predators.

Evolutionary and ecological factors

Bird migration is a flexible behavior that has developed in many bird groups. Scientists think genes help birds migrate, but some birds can learn to migrate without genetic changes. This helps explain why 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 can cause timing problems. 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 being seen in many bird species in 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. While some diseases 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 study bird migration in many ways. One old method is marking birds, like putting a small mark on a swan's beak. Now, scientists use radar and satellite tracking to watch birds fly. They also use tools like the Emlen funnel to learn how birds find their direction.

Other ways to study include looking at special materials in bird feathers to find where birds go and listening to bird calls at night with microphones. These methods help us learn more about bird migration.

Threats and conservation

People can hurt birds that fly long distances. Because these birds travel through many countries, protecting them needs help from many nations. Some agreements, like the Migratory Bird Treaty Act of 1918 in the US and the African-Eurasian Migratory Waterbird Agreement, work to keep birds safe.

Birds face dangers during their trips. Hunting in places like Afghanistan and Central Asia has hurt some birds, like the Siberian cranes. Things like power lines, wind farms, and oil rigs can also be risky. Pollution, storms, and losing places to rest and eat are other problems. Many spots where birds stop have been lost because of farming.

People are working to help birds. In California, farmers fill their fields with water in winter to make temporary wetlands. This gives birds like the White-fronted Goose a place to rest and eat. This also helps farmers because they need less fertilizer. But if too many birds are in one spot, it can hurt water quality, so it’s important to spread out these places for birds.