Bird Migrations

Podcast Transcript

Every year, hundreds of millions of birds around the world migrate.

Some migrate short distances, some migrate incredibly long distances, and others don’t bother to migrate at all.

For centuries, people didn’t know why birds migrated, how they managed to travel such long distances every year, or where they went.

Thanks to modern science and technology we now have a much better idea of how it works and where they go.

Learn more about bird migrations on this episode of Everything Everywhere Daily.

Bird migrations—assuming the internet theory is wrong, and birds actually exist—are one of the most incredible navigation feats in the natural world.

Before I get into the details of the hows and whys, I should probably first explain the what.

Migration is nothing more than the seasonal movement of animals, almost always during summer and winter. It is almost always between areas used for breeding and/or feeding.

Birds are not the only animals that migrate. There are other animals that will migrate, such as wildebeest, caribou, whales, and butterflies. However, none of them are able to perform the migratory achievements that birds can.

For the longest time, humans had no idea that birds migrated. Because people couldn’t travel very far, they only saw one-half of the migration story. They knew that birds disappeared at some point in the year and later they came back.

Where they went and why they left was hotly debated.

Aristotle had several different views on what happened to birds when they disappeared.

He thought that swallows hibernated in the mud during the winter months. He thought that redstarts turned into robins in the winter.

On the same note, Aristotle also knew that cranes were reported to fly from north of the Black Sea to the marshes along the Nile every year.

Not everyone was confused about birds’ movements. The ancient Polynesians used bird migration and the direction in which they flew to help with their navigation.

Nonetheless, Aristotle’s theories persisted into the Middle Ages and beyond, even when they were incorrect.

Well into the 18th century, most Europeans assumed the swallows hibernated for the winter, even though there was no evidence of swallows actually hibernating.

Reports from sailors began to change the minds of naturalists. They reported seeing swallows flying north over the Mediterranean.

In 1822, a stork was found in Germany that had been shot with an arrow made out of an African wood. The only way it could have gotten there is if it had been shot in Africa, thousands of miles from where it had been found.

After sufficient observations of birds in different parts of the world at different times of the year, bird migration was universally accepted by the mid-19th century.

The migration of birds is an evolutionary strategy that has developed over millions of years.

There are multiple reasons for migration, and there are some drawbacks to the strategy as well.

The most obvious reason for migration is food. Many birds migrate to areas where food is abundant, especially during the breeding season. For example, insectivorous birds travel to northern latitudes in spring and summer when insects are plentiful, then return to warmer climates when food sources in the north diminish in the fall and winter.

Bald Eagles will migrate, but not nearly as far as other birds. They will spend summers in the far north and fly south just far enough where ice doesn’t form in the winter. Eagles can tolerate reasonably cold temperatures, but they need to eat, and they can’t do that if lakes and rivers are inaccessible.

Other birds do migrate just because of temperature. They never developed the ability to insulate themselves in cold climates so they have to leave.

A major reason for migration is breeding. Birds have certain nesting areas that they fly to every year. At least in one case, the fact that a bird species migrated to a protected area for breeding might have saved them.

The whooping crane was once abundant in North America, but due to habitat loss, its population decreased to the point where it almost became extinct.

However, a small population managed to survive, but no one knew where their nesting area was.

In 1954, they found where the whooping cranes were hiding out. The only remaining natural breeding habitat for the species was found in Wood Buffalo National Park in northern Alberta, Canada.

Wood Buffalo is a very large national park, the largest in North America, and very inaccessible. Most of the park is wetlands that can’t be accessed during the summer months.

The whooping crane was migrating there in the summer to breed in an area where they were completely protected. The native people who lived there didn’t even know that the cranes were there because they could only access the area in the winter when the water was frozen, and the cranes were gone.

Once it was accepted that some birds underwent a seasonal migration, the really big question was how. Naturalists eventually realized that some species of birds were migrating over an enormous portion of the Earth every single year.

Bird migration isn’t perfectly understood, but there are several popular theories as to how they manage to navigate long distances.

The first is endogenous programming.

Endogenous programming in bird migration refers to the innate, genetically encoded behaviors and physiological processes that guide birds in their migratory activities. This internal programming dictates when birds migrate, where they go, and how they get there, independent of environmental cues.

Most birds possess an internal biological clock, or circannual rhythm, that informs them of the optimal times to begin their migration. This clock is influenced by hormonal changes, which are triggered by variations in daylight length. These hormonal changes can affect behavior and physical conditions, such as fat storage, to prepare them for migration.

Much of the migratory instinct in birds is coded into their genes. Research has shown that migratory behaviors are heritable. Studies involving crossbreeding of migratory and non-migratory birds have indicated that offspring inherit migratory directions and distances, suggesting a strong genetic component to migration patterns.

However, many ornithologists think that there is more to it than that. That there are built-in mechanisms that assist birds in their navigation.

Some birds have tiny magnetic particles, often the iron oxide called magnetite, in their beaks or brains. These particles are thought to function as tiny compasses. Research suggests that these particles could help birds detect Earth’s magnetic field, providing a sense of direction. This is known as the magnetite-based hypothesis.

Another prominent theory involves cryptochromes, which are light-sensitive proteins found in the retinas of migratory birds. These proteins might enable birds to literally see Earth’s magnetic field as a visual pattern overlaying their normal field of view, often hypothesized as a visual “map.” This light-dependent mechanism is believed to be influenced by the wavelength of light, particularly under blue light.

However, birds sense magnetism, it is probably more sophisticated than simply knowing which way is north and south.

Unlike a compass, which shows the magnetic pole as a horizontal attraction pointing north and south, birds are sensitive to the inclination of magnetic lines of force. This means they can detect the angle at which these lines intersect with the Earth.

One reason we know it can’t just be Endogenous programming is because of experiments where the magnetic field around birds was artificially altered. These experiments show that changing magnetic conditions can confuse migratory birds, affecting their ability to orient correctly.

This strongly supports the theory that birds rely on Earth’s magnetic field to navigate.

Most birds will migrate in flocks. There are several good reasons for this. The first has to do with energy conservation.

When geese, for example, fly in a V formation, they will use about 20% less energy than they would if they flew alone. This is the same principle as to why cyclists drag behind a leader who will take most of the wind resistance.

The other reason is safety in numbers. If you fly in a group, you are more likely to have some of your group survive than if you were to travel solo. You also just might dissuade a predator from attacking if you have a large enough group.

As strongly encoded as the desire to migrate is amongst birds, there has been a recent phenomenon of birds who are no longer migrating. This has happened with some populations of Canadian geese.

Geese in urban areas have started staying put during the winter rather than flying south.

Urban settings often provide plentiful food sources, such as grass from lawns and parks, and artificial bodies of water that do not freeze over in the winter, like heated ponds in golf courses and parks. These conditions reduce the need for migration.

The problem is that as the geese stay put, their offspring may lose their knowledge of migration, meaning that they might stay put forever.

As I mentioned earlier, every species of bird that migrates has a totally different migration pattern.

One of the most impressive migrations of any species has to be the Arctic Tern. It has the longest migration of any bird in the world, traveling about 71,000 kilometers or 44,000 miles round-trip each year between its Arctic breeding grounds and the Antarctic coast.

Arctic terns are a circumpolar species, which means they are found all over the Arctic in North America, Asia, and Europe. As such, they take multiple routes to fly to Antarctica each year.

Tagged birds have found that have flown acoss oceans east-west in the the course of migrating from the Arctic to the Antarctic and back.

By contrast, the bird recognized as having the shortest migration is the North American blue grouse, also known as the Dusky Grouse. They live in the American and Canadian west in the Rocky Mountains. They are ground-dwelling birds that are only capable of very short flights.

They only migrate a few hundred meters every year up and down a mountain. Oddly enough, they go up the mountain in the winter, not down.

What do you do if you are a bird species in eastern Asia and your southern flight south is blocked by the Himalayas, the highest mountain range in the world?

Bar-headed geese just fly over them. They are known for flying over the Himalayas during their migration, reaching altitudes of up to 6,500 meters or 21,300 feet, where oxygen levels are significantly low.

The blackpoll warbler, a small North American songbird, performs one of the most grueling non-stop flights of any migratory bird, traveling over 2,700 miles or 4,350 kilometers from New England to South America without stopping.

It is suspected that they feed on insects while flying.

Not every bird that travels long distances is migratory. Some birds are simply nomadic and move from place to place whenever conditions merit a move.

Flamingos and albatrosses are two types of birds that can travel thousands of miles but do not technically migrate. They are traveling all the time and will up and move, usually in an east-west direction, whenever food becomes scarce.

Bird migrations are one of the greatest wonders of the natural world. Through a combination of genetic programming and the ability to read magnetic fields, these descendants of dinosaurs are able to pull off some of the most impressive navigational fetes in the animal kingdom