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Animal Migrations

Learn about some animals that migrate, and the surprising lengths they travel. Sorry about the audio quality. I probably should have rerecorded the whole thing.

(I have unlocked this for everyone to listen to and while I was doing that, I added a transcript of the show.)

Show transcript:

Everyone knows that birds migrate, some mammals migrate, and even the monarch butterfly migrates. But how much do you actually know about migration?

If you’re like me, not much. So let’s learn about migration!

Lots of birds migrate, from geese and ducks to tiny little hummingbirds. Most species that migrate live in the northern hemisphere and move south to warmer areas for winter, where food is more plentiful, and return north again to breed during the summer since summer days are much longer in northern regions. In the southern hemisphere, birds are often partial migrators, with some populations staying put all year round, while other populations migrate. Birds that usually live in the mountains may migrate to lowlands for the winter.

Migrating birds travel along routes known as flyways, which often follow rivers or mountain ranges. They usually don’t fly at high altitudes but some species that cross the Himalayas during migration have been verified as flying at more than 21,000 feet, or 6500 m. That’s four miles above sea level, or over six km. People climbing Mount Everest have found skeletons of ducks on the Khumbu Glacier.

People used to think that birds hibernated during the winter, which is why they weren’t around, or that they flew to the moon for the winter. Some people thought birds turned into other animals during the winter, like mice, which doesn’t actually make any flipping sense at all. It wasn’t until the 18th century that scientists started to figure it out from observations of migrating birds and one very interesting stork.

The white stork is a large, beautiful bird, white with black markings on the wings and red legs and bill. It stands about four feet high, or 125 cm, with a wingspan of about seven feet, or over two meters. It eats fish, insects, and small animals and other birds. In the summer it lives in parts of Europe, but it migrates to parts of Africa. It’s the stork that’s supposed to bring babies to expectant parents.

While scientists were still arguing over whether storks and other birds hibernated or migrated for the winter, someone shot a white stork in the German village of Klütz in May of 1822 that had an arrow stuck through its neck. And not just any arrow! It was a type of slender spear made by people in central Africa. The bird was taxidermied and mounted with the arrow still in place, and is now in the University of Rostock’s collection. Imagine getting stabbed by a spear that stuck in your neck, and not only surviving, but flying with that spear in your neck for hundreds of miles. I bet it hurt like crazy.

The longest bird migration we know of is the Arctic term, which travels from the Arctic to breed to the Antarctic to not breed. That’s more than 12,000 miles, or 19,000 km. But the Arctic term spends most of its time on the wing, and is an amazing long-distance flyer even when not actually migrating. A study in 2010 attached tracking devices to eleven birds, and the average distance traveled in one year was 44,100 miles, or 70,900 km. And this is a bird that can live for 30 years. But the bar-tailed godwit flies from Alaska to New Zealand without stopping. That’s 7,000 miles, or 11,000 km, that they travel in eight days.

So we know why birds migrate, but how do they do it? We’re still learning how it works, but it seems to be a combination of factors, and the combination is different for different species. Some birds have the ability to detect the earth’s magnetic field, some birds use the sun as a compass, some navigate by the stars, some use visual landmarks and even smells. The basic need to migrate is innate, but researchers have found that young birds get lost more easily on their first migration and in later years do a better job getting to the right place as they learn the route.

There’s something really neat about birds that can sense earth’s magnetic field, by the way. We still have a lot to learn about it, but recent research suggests that the bird can actually see the magnetic field. That is so awesome. I bet it’s a really cool color, one we can’t even imagine.

Penguins migrate too, usually swimming part of the way and walking the rest. One population of Adelie penguins from Ross Island in the Antarctic travels over 5,000 miles, or 8,000 km, to reach its breeding grounds. Emperor penguins walk over 100 miles, or 160 km, to their breeding rounds.

That brings us to whales, because that just makes sense, like penguins are tiny bird-whales?

Baleen whales are migratory, swimming from the cold waters where they spend most of their time to warmer waters to give birth. Whale calves are born with only a thin layer of blubber so they need to spend their first months in warm water until they build up protection from the cold. Blue whales travel from the Arctic and Antarctic to the tropics to have their babies, which takes them a couple of months, during which time they eat almost nothing and live off body reserves. Only female right whales migrate, while both males and females of other baleen whales migrate. The longest route is that of the humpback whale, which travels over 3,000 miles, or 5,000 km, to its calving waters. Toothed whales don’t generally migrate the way baleen whales do, although they do move around a lot as they hunt for food.

A lot of land mammals migrate, especially hoofed mammals. The great wildebeest migration in Africa is famous for the enormous herds that travel more than 500 miles, or 800 km, between Tanzania and Kenya. Something like one and a half million wildebeest travel in vast herds, along with other antelope species and zebras. They calve in the Serengeti. Lions and other predators follow the herds, picking off injured and exhausted animals. Wildebeest like the Serengeti because the grass that grows there is short, so it’s harder for predators to sneak up on their calves. But the Serengeti doesn’t get much rain most of the year. The herds have to keep moving to new pastures before the rivers and water holes dry up.

Some bats migrate too. Some populations of Mexican free-tailed bat migrate north from Mexico and Central America to Texas and other western United States during the summer, although some other populations don’t migrate at all. Several other bat species from North America migrate south for the winter. The straw-colored fruit bat in Africa migrates in huge numbers, ten million little bats flying more than 1,200 miles, or 2,000 km, to follow the seasonal ripening of fruit.

Mammals and birds aren’t the only migrant animals, of course. Salmon migrate from the ocean up rivers to spawn and die, while eels migrate from rivers to the ocean for the same purpose. The young salmon and eels make the reserve trip. Many snake species migrate to shared dens for the winter, like rattlesnakes. It turns out that rattlesnakes are social animals during the winter, and family groups gather together in the dens, with sisters being especially close.

The leatherback sea turtle migrates from the northeastern coast of North America to tropical areas, including South America and Africa. Even some crabs migrate. The Christmas Island red crab only travels five miles inland, or eight km, but that distance takes it a month and it’s as perilous in its own way as the wildebeest’s vast trek. It even climbs down a cliff 40 feet tall, or 12 meters.

Monarchs aren’t the only butterfly species that migrate, and they aren’t the only insect that migrates either. The globe skimmer, a type of dragonfly, migrates 600 miles, or 1,000 km, over the ocean from southern India to Africa. But the monarch butterfly’s migration is pretty spectacular, so I’ll finish with that one.

The idea that butterflies migrate just blows my mind. I remember sitting outside my middle school one crisp fall day, waiting for my mom to pick me up, and watching monarch butterflies fly by, one every few minutes, all of them heading steadily in the same direction. I didn’t know then that they were migrating.

The monarch is a modest-sized butterfly, with orange and black wings with white spots along the edges. It lives in many parts of the world, but only the North American subspecies of monarch migrates. Every autumn, monarch butterflies living in North America, where they breed, head south to winter in central Mexico, a trip that can be as long as 3,000 miles, or 4,800 km. They spend the winter in oyamel fir trees, millions of butterflies in the branches. When spring arrives, the butterflies head north, but they don’t get all the way back to their original range. If they’re lucky, they reach Texas, where they mate and lay eggs on milkweed plants before dying. The caterpillars hatch, eat up the milkweed, spin a cocoon and emerge transformed into new butterflies that continue the flight north, deeper into North America. But those butterflies don’t make it all the way to their parents’ home range either. They too stop to mate, lay eggs, and die. It can take four or five generations for monarch butterflies to reach Canada and other distant parts of North America, and by that time it’s autumn again. The butterflies fly back to Mexico.

Butterflies heading north live out their entire life cycle in only five or six weeks, but the butterflies that return to Mexico live up to eight months. Researchers think the northward migration follows the blooming of milkweed plants. Milkweed contains toxins that make the monarchs poisonous to a lot of animals, but some birds and a lot of insects will eat the caterpillars.

The North American monarch is declining in numbers, mostly due to the decline of milkweed. The best way to help the butterfly is to plant milkweed in any area you don’t want to mow very often.

Think about insects that lived back in the days of the dinosaurs and earlier. We don’t know much about them. We certainly don’t know if any of them migrated. In the future, long after humans are gone and the monarch butterflies are gone too, no one will know they migrated. But you know.

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