Deep Dive
Shownotes Transcript
Every year, hundreds of millions of birds around the world migrate. Some migrate short distances, some migrate incredibly long distances, and some 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. But thanks to modern science and technology, we now have a much better idea of how birds migrate and where they go. Learn more about bird migrations on this episode of Everything Everywhere Daily.
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Learn more at shopify.com slash enterprise. Bird migrations, assuming the internet theory is wrong and birds actually do 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 the summer and winter. And 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 wildebeests, 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 then 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. And he also thought that redstarts turned into robins during the winter. And I should also note, Aristotle knew that there were cranes that were reported to fly from the north of the Black Sea to the marshes along the Nile River every year.
Not everybody 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 though they were incorrect. Well into the 18th century, most Europeans assumed that the swallows just 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 had reports of seeing swallows fly north over the Mediterranean. In 1822, a stork was found in Germany that had been shot with an arrow made out of 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. And there are multiple reasons for migration, and there are some drawbacks to this 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, insectivore birds travel to northern latitudes in the spring and summer when insects are plentiful, and then they 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 the 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 the lakes and rivers are inaccessible.
Other birds do migrate just because of temperature. They never develop the ability to insulate themselves in cold climates, so they have to go someplace warm. A major reason for migration is breeding. Birds have certain nesting areas that they fly to every year. And at least in one case, the fact that a bird species migrated to a protected area for breeding may 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 nobody 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 it's very inaccessible. Most of the park is wetlands and 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's surface every single year. Bird migration isn't perfectly understood, but there are several popular theories as to how they managed to navigate long distances. The first has to do with endogenous programming. Endogenous programming in bird migration refers to the innate, genetically encoded behaviors and psychological 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 shown that offspring inherit migratory directions and distances, suggesting a strong genetic component to migratory patterns. However, many ornithologists think that there's more to it than that, that there is a built-in mechanism that assists birds in their navigation.
Some birds have tiny magnetic particles, often an iron oxide called magnetite, in their beaks or their 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 the 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's probably more sophisticated than just 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 force lines. This means that they can detect the angle at which these lines intersect with the Earth. One reason we know it can't just be endogenous genetic 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 the Earth's magnetic field in order to navigate. Most birds will migrate in flocks, and 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're more likely to have some member of your group survive than if you were to go 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. And this has happened with some populations of Canadian geese. Geese in urban areas have been 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 and golf courses and parks.
These conditions reduce the need for migration. The problem is that as these geese stay put, their offspring may lose their knowledge of migration, meaning that these populations may stay put forever. As I mentioned earlier, every species of bird that migrates has a totally different migration strategy. 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 that they're found all over the Arctic in North America, Asia, and Europe. As such, they take multiple routes to fly to Antarctica every year.
Tang birds have been found that have flown across the oceans east to west in 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're 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. And oddly enough, they go up the mountain in the winter, not down. What do you do if you're a bird species in Eastern Asia and your southern flight is blocked by the Himalayas, the world's highest mountain range? Bar-headed geese just fly over them.
They're 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 black pole 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 ever stopping. It's suspected that they feed on insects while they're flying. And I should note that 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 for thousands of miles but do not technically migrate. They're traveling all the time and will usually move 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 feats in the animal kingdom. The executive producer of Everything Everywhere Daily is Charles Daniel. The associate producers are Benji Long and Cameron Kiefer.
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