Why Do Birds Migrate?

Have you ever noticed how much more abundant birds are in the summer than in the winter? Starting in the spring, many birds arrive back at their breeding grounds in North America to enjoy the warm weather.

Bird migration is one of the most fascinating aspects of bird biology. Their physical ability, regularity, and navigational skills are truly remarkable. But why do birds put their bodies under so much stress to fly such great distances each year?

Birds migrate when food resources become scarce during the fall and winter months. Northern summers can be ideal for birds, so much so that many migrants choose to breed there.

In the winter, however, conditions become so harsh that birds must fly south to avoid the harsh winter months. The same general pattern is seen across the world and involves a huge variety of different bird species.

A flock of flamingos migrating in flight

The earth experiences four seasons each year, and the variation between the seasons increases with latitude. In other words, the further you are from the equator, the greater the difference between seasonal temperatures.

The earth is tilted as it orbits the sun so that the north pole is closer to the sun in June while the south pole is closer to the sun in December. This is what causes the seasons and the reason why it is winter in South America when it is summer in North America.

Birds can live in perpetual summer by flying over the equator and into the southern hemisphere. Nevertheless, such lengthy migrations aren’t always necessary. In fact, many birds migrate entirely within the United States and Canada, sometimes without even leaving their home state.

Migration is very dangerous, and many birds do not survive the journey. Predators, hunters, weather events, and exhaustion all take their toll, and yet birds continue to migrate like clockwork year after year.

Keep reading to learn more about the magic of bird migration.

Close up of an Osprey in flight

How do birds prepare for migration?

Long-distance migration is not something that birds can jump into without preparation. Some birds migrate slowly, stopping frequently to rest and feed along the way, while others, like barn swallows, can easily feed on the wing. There are birds that complete their entire migrations in a single flight, however, even flying over oceans where rest is impossible.

Read on to learn how birds prepare themselves for long migrations.

Energy storage and diet changes

Long-distance migrants must build up sufficient fat stores to provide the energy they need for their journey. In fact, some birds can literally double their normal body weight in preparation for their migration.

They do this by increasing their food intake and also by focusing on foods that they can convert into fat. This isn’t something that birds have to think about consciously. Instead, hormones trigger them to start increasing their food intake.

Birds don’t only eat more food, however, they also change their diets in many cases. Insect-eating songbirds, for example, often begin to focus on fruits because they can be converted into fat reserves.

Organ and muscle changes

Some birds take migration preparation to pretty extreme levels, even partially shrinking and reabsorbing some organs to make room for the development of other parts of the body.

The bar-tailed godwit is the best-known example of this incredible preparation. These birds grow their hearts and flight muscles while shrinking their digestive systems.

They need to do this to get themselves into peak physical condition to make a non-stop 7,000-mile flight over the Pacific Ocean from Alaska to New Zealand.

Molting

Migratory birds molt their feather before beginning their journey. This typically happens right after they have finished breeding. Having a fresh new set of feathers makes migratory birds more efficient at flight.

A swallow in flight

How do birds navigate?

Bird migration can be a grueling feat, taking species as small as rufous hummingbirds thousands of miles to and from their breeding and overwintering grounds each year. The ability to return to the same areas year after year is just as astounding as the physical endurance required to get the birds there!

Read on to learn some of the amazing ways that birds navigate while on migration.

Instinct and learning

Birds are thought to use both instinct and perception to navigate to and from their overwintering grounds. Some bird species learn the route to their overwintering grounds by following their parents, while others have to rely on instinct to make their first journey.

The simplest methods of navigating are seen in first-time migrating birds that know to fly in a certain direction and for a certain amount of time. They are thought to learn certain landmarks and perhaps even smells along the way, helping them develop a mental map that will help them return to the same site year after year.

Magnetic fields

We have long known that birds are able to use the Earth’s magnetic field to determine direction. This amazing sensory ability is known as magnetoreception.

Many of the finer details remain a mystery, but scientists have discovered that birds are able to ‘see’ magnetic lines. The stimulus is sent to the brain through the eyes via the optic nerve.

Birds also use magnetoreception in another fascinating way. They are also thought to detect magnetic intensity thanks to signals sent from a part of the beak containing magnetite. This might allow birds to make up a map based on the varying magnetic intensity of the world around them.

Celestial orientation

Birds are able to use the position of the sun as a compass to guide their movements. Of course, the sun does not appear to remain in the same position throughout the day so birds must also factor in the time of day and even year to some extent when navigating by the sun.

The sun is a great guide during the day, but what about birds that migrate at night? Clever experiments have shown that birds are able to use the position of stars to navigate while on migration.

They use the position of stars that form the axis of the night sky to navigate, rather than by learning constellations.

Bar-tailed Godwit in flight

Do all birds migrate?

Most bird species do not migrate. Depending on the climate and latitude where you live, there could be more or fewer migratory birds than resident sedentary birds.

Winters are too harsh to sustain most bird species in the Arctic and northern temperate regions, so most birds visit these areas in the summer. In more tropical regions, however, the variation between the seasons is less severe, and many birds are able to stay put throughout the year.

Common Sandpiper

Why do some birds migrate and others don’t?

Many birds live in areas where the environment remains suitable all year long. A bird has no need to migrate if it is able to withstand the prevailing weather conditions and has access to a steady supply of food and water all year long.

Some birds, like northern cardinals, are able to survive pretty far north without migrating. They are adapted to endure the cold weather and forage for the seeds and plant material they need to survive.

Cardinals can stick around because food resources are always available, but what about birds that feed on seasonal foods like insects? These birds, which include many songbirds like warblers, are forced to move south to warmer areas where insects remain active.

Close up of a perched male Northern Cardinal

How far do birds migrate?

Birds migrate different distances depending on their species and where their breeding and overwintering grounds are.

Some species migrate only a few hundred miles or so, while others are capable of incredibly long flights, exceeding twelve thousand miles. The Arctic tern is known to have the longest migration of any bird.

The two most common forms of migration in the bird world are latitudinal and altitudinal migration. Latitudinal migrants usually travel north and south as the seasons change. These migrants are known to travel the longest distances, often covering several thousand miles every year.

The peregrine falcon is another great example of a long-distance migrant. These birds can travel an amazing 9,000 miles between Canada and Argentina each spring.

Altitudinal migrants tend to travel much shorter distances. These birds move from higher mountains in the summer, to the lower lying areas in the winter. Altitudinal migrants might only travel a few hundred miles, and they can travel in any direction.

Costa’s hummingbirds are a good example of a short-distance migrant. These tiny birds migrate between the deserts of the American Southwest and the coastal areas of the West Coast, sometimes without even leaving the state of California.

A pair of Arctic Terns at their breeding grounds

An unproven theory

Not all bird species migrate – in fact less than half of them do. Modern patterns of migration are also adapting due to climate change, with the timing of departure seemingly under flux as a result.

If we consider other historical epochs when the earth’s climate changed, during ice ages for example, then another perspective arises for our consideration. As the poles got colder, it is likely that birds moved closer to the equator. As the ice age subsequently receded, millennia later, it is equally likely that some ventured back further north or south again.

The ancestors of many of the birds now familiar to us may not have otherwise have had any cause to visit the areas they now inhabit for parts of the year and we assume to be their natural home.

We also know from skeletal remains that those ancestors were established far earlier in history in what we now regard as far-flung places and that, in evolutionary terms, they are relatively recent visitors to the major areas of human population.

Given that nature has many examples of natal philopatry, that being the phenomena where adults eventually return to the place they were born to reproduce, such as salmon, eels, turtles and others, could it be that the swallow, the osprey, the goose and the sparrow are not, as we see it from our human perspective, leaving but returning - to their ancestral homes – until in the distant future the climate changes sufficiently once again and they can remain there all year?

From an avian perspective, is their presence in our neighbourhoods merely an inconvenience to be abandoned once the next interglacial period begins, as it inevitably will?

Osprey in flight