To The Moon and Back
If you asked a European person in the 1700s “Where do birds go in the winter?” they might just look at you with a blank stare, then shake their head and walk away.
Or, feeling more helpful, they might refer you to a pamphlet published in 1703, succinctly titled “An Essay Toward the Probable Solution of this Question: Whence Come the Stork and the Turtledove, the Crane, and the Swallow, when they Know and Observe the Appointed Time of their Coming.”
The answer given in the pamphlet? Birds fly to the moon in the winter.
Silly, right? Well, sure, but before scientists had studied birds (and the moon) more carefully, this would have been about as reasonable as any other answer. Another explanation in ye olden days was that birds hibernate at the bottom of the sea or while buried in swamp mud.
But in 1822, a hunter in Germany shot and killed a White Stork and found that the bird already had a large arrow lodged in its neck. And it turned out that the arrow came from Central Africa.
This lucky/not-so-lucky stork gave the world its first solid piece of evidence that birds in the northern hemisphere often undertake long journeys south in the winter.
This bird was dubbed a pfeilstorch, which means ‘arrow stork’ in German.
Amazingly, this bird was not one-of-a-kind. About 25 of these pfeilstorchs have turned up since 1822. Those are some pretty hard-core birds. To survive getting impaled with an arrow and then flying thousands of miles north with the arrow stuck in your body? Crazy.
These days, we need not puncture birds with arrows to understand their annual movements, thankfully. We now have amazing tools like GPS trackers that provide precise location info without any harm to the animals.
We live in a time when research on bird migration has given us amazingly detailed pictures of just where birds are going and how they get there.
Migration is one of the most amazing things about birds—their seemingly miraculous feats of orientation and endurance on long-distance, annual flights.
This is a big topic, so I can’t do a deep dive into “all things migration” here. Instead, this will be an overview of migration in birds.
Birds are incredibly mobile beasts, given that they fly. Compared to us lowly Earthbound creatures, birds are free to go where they please. But they don’t just fly around the planet aimlessly.
Instead, a range of distinct movement types and patterns are seen among the Earth’s approximately 10,000 bird species.
At the smallest geographic and temporal scales, birds move around in their local habitats over the course of a day. They forage for food, seek shelter from predators and the weather; they defend territories, look for mates, and tend to their nests.
Another type of movement is the one that every bird makes once in their lifetime. Young birds begin their lives in one location and then venture forth into the unknown, often on their own, away from where they were born. This movement is called dispersal. Some bird species disperse long distances, others stay very close to where they hatched.
Humans disperse, too. I live in Oregon now, but I was born in California, where I left my dear old Mom. Some other guys in their forties might still live with their moms. Those are cases of non-dispersal.
And then we have migration, which is our topic today. This is the regular, seasonal movement undertaken by birds. In most cases, it goes from north to south, and vice versa. Migration, as you’ll see, comes in an assortment of flavors.
Types of Annual Movements in Birds
Migration is an annual movement, repeated every year. But it’s not the only type of annual movement in birds.
One type of annual movement is that of no movement. Otherwise known as residence.
Most of the world’s bird species—perhaps as much as 80%—don’t technically migrate. Most just hang around in their usual haunts, no matter what the season is. They may have a particular season for breeding, but they don’t travel very far to take care of that business. We call these homebody birds ‘residents.’
Most bird species in the tropics are residents. And since most of the world’s bird diversity is concentrated in the tropics, it isn’t surprising that the bird species showing true migration are actually in the minority at the global scale.
So we have resident birds, but the other 20-40% of bird species are not residents—they go through some kind of migration.
The first type of true migration to discuss here is facultative migration. This is where a bird species migrates in some years, but not in others. Birds of this sort may spend the entire year in one location when conditions are normal and good. But if conditions get lousy in a given year… if, for example, there’s a harsh winter and/or food is in short supply, individuals may then choose to migrate some distance—perhaps even a long way—to greener pastures. “Choose” is the keyword here. These birds assess the local, current conditions and decide to stay put or migrate. Species with facultative migration often depend on food resources that can vary widely in availability from one year to the next.
For example, migration in Pine Siskins is erratic across the years, depending on where their food is. These little birds sometimes show up in unexpected places in years when there are failures of cone crops and other seeds in the Siskins’ normal winter range. In other places, they don’t migrate very far, if at all, when food is plentiful. These boisterous members of the finch family are facultative migrants. Also, I love the scientific name for this species: Spinus pinus.
Then we have obligate migration, which is probably what many of us think of when we hear the word “migration.” This is where a species migrates predictably between two regions every year.
Obligate migrants spend part of the year in a breeding range, and the rest of the year in a non-breeding range, what is for some species called a ‘winter range.’
For an obligate migrant species, all or nearly all individual birds make the annual journey there and back again. They’re hard-wired to migrate. They are obligated by their genes to make their annual journeys.
There are many familiar birds that are obligate migrants: Swainson’s Hawk is a North American raptor that spends the non-breeding season in the pampas grasslands of southern South America. Almost every individual Swainson’s Hawk makes the epic round-trip journey of over 12,000 miles to South America every year.
A European example of an obligate migrant is the Garden Warbler, which flies to southern Africa every winter, more or less like clockwork.
Another form of annual movement is nomadism. This is where members of a species move about over the course of a year in a much less predictable pattern. These birds are scouring the landscape for some kind of vital resource that is only sporadically available. This resource is usually food. This food could be local infestations of insect prey or a plant species that has boom and bust cycles of fruit or seed production.
Nomadism isn’t technically migration because these movements aren’t predictable—they change from year to year and are entirely dependent on the location and availability of that key resource.
The Red Crossbill is a classic example of a nomadic species across the Northern Hemisphere. Flocks of crossbills criss-cross vast areas of coniferous forest as they look for cone crops.
Perhaps not surprisingly, many desert birds are nomadic. For example, the Gray Teal, a type of duck, is nomadic in the arid parts of Australia, moving from place to place in response to water and food availability.
Most of the bird species with true migration are not obligate migrants across their entire range. Instead, they display different migration types in different geographic areas. Such a species might have some resident populations, while also having populations that migrate, either facultatively or obligately. We call this partial migration. Where populations of a single species display different movement types across the species’ range.
Familiar species that are partial migrants include Bewick’s Wren in North America and the European Robin and Eurasian Kestrel on the other side of the Atlantic.
Why Do Birds Migrate?
Why do birds migrate? Why go through all the trouble?
This is a big question and we don’t fully understand all the ins and outs of how or why migration evolved in birds. It’s complex.
But it’s reasonable to assume that migration must improve the chances of survival and reproductive success of the species that migrate. Otherwise, it wouldn’t have evolved in the first place.
The best explanation for why migration evolved in birds is that it allows them to take advantage of seasonal differences in environments around the world.
Some places on Earth are great for mating and raising a family, but only at certain times of the year. Other places are better for finding plenty of food and shelter during the non-breeding season.
This is all about tradeoffs and optimization. Migration has enormous costs and risks involved, so these have to be offset by significant benefits—at least on average, in the long term.
Now, if you’re like me, you sometimes lie awake at night thinking about small songbirds that lived 15 thousand years ago in Equatorial Africa.
Let’s think about such a bird species now as we ponder how migration might have evolved since the end of the last Ice Age.
These birds might have made short-distance annual movements across their tropical homelands, looking for seasonably variable food.
As the planet warmed, and glaciers melted away across Europe, vast new lands opened up to the north. Some of these little birds ended up exploring the newly ice-free Europe. There, they found little competition for nesting space or food resources. Predators and diseases were few. And the summer days were extra long.
These pioneer birds had advantages that allowed them to produce more offspring than their friends who never left central Africa. Natural selection favored the birds that migrated back and forth between the two continents and, eventually, the species became fully migratory.
Such a scenario may or may not have been how it all went down. Some researchers have made the case that migration evolved most recently in northern birds who expanded southward, rather than the other way around, as in our little story.
And, most likely, the ability and drive to migrate long distances didn’t originate only 15 thousand years ago. Scientists think migration goes way back, millions of years, and is likely a primitive characteristic in birds. Migration behavior has probably been lost and resumed across many bird lineages, again and again, as the world’s environments have continued to change.
Different Patterns of Migration
True migration comes in a few flavors. Some major patterns of migration can be seen across the avian world.
First off, migration distances vary among species. There are short, medium and long-distance migrants. The range here is huge, with some birds making annual migrations of only a few tens of miles while others literally travel from one end of the Earth to the other.
Another way that migration varies among species is by pacing. We have “hops, skips, and jumps.”
Birds that ‘hop’ are the ones that assume the most leisurely pace, making many brief stops on their migration route. Those that ‘skip’ make fewer stops, maybe three or four. And birds that ‘jump’ are fast-paced migrants, resting at only one or two places along the way.
Swainson’s Thrush (named for the same English ornithologist as the hawk I talked about earlier) is an example of a ‘hopping’ species. This bird makes a series of brief stops on its migration, usually lasting from 1 to 4 days.
At the other end of the spectrum we have species known to jump. The Bar-tailed Godwit is a champion jumper. These crazy shorebirds can fly nonstop from Alaska to New Zealand in only 9 days. That’s a distance of over 6,000 miles!
Many species have a one particular migration pathway in the spring, as they fly toward their breeding areas, but then they travel a different path in the fall. This is common in songbirds. When you look at the migration paths traveled over the course of the year for one of these species, you see that they form a loop.
Why would birds travel different routes as they come and go? The best explanation is that because there are seasonal differences in weather and wind conditions at different places, it makes sense to follow the path of least resistance in each season. That is, the path where you’re more likely to have a tailwind and good weather.
These loop migration paths could also come from food availability, which can vary by season and location.
The feisty little firecracker known as the Rufous Hummingbird displays a looping migration pattern. In spring these hummers fly north from Mexico to the Pacific Northwest, following the Pacific Coast. Then they work their way back down to Mexico in the fall, this time through the Rocky Mountains, where they refuel on nectar from late-blooming mountain wildflowers.
All the movement we’ve been talking about so far has been in the horizontal dimension, across latitudes and longitudes. But for some bird species, annual movements in the vertical dimension are the most significant. This is what we call Altitudinal Migration.
Altitudinal migrants move up-slope to higher elevations at one time of the year and then back down to lower elevations for the rest of the year.
Birds might do this to reach high-elevation habitats accessible in only the warmer months. Or they might take advantage of different food resources that vary by elevation and season. Altitudinal migration is more common among species in the tropics, and food availability is usually the driver in those regions.
Two northern latitude examples of birds with altitudinal migration are the Varied Thrush in North America and the Wallcreeper in Eurasia. Both species live up in the mountains in the warm months, but retreat to lower elevations in the winter.
Not all members of a species do exactly the same thing every year. There’s individual variation—some wiggle—in the migration paths taken. Also, distances travelled often vary between males and females, and between juveniles and adults. For example, in Dark-eyed Juncos, adults migrate farther than young birds and females migrate farther than males. The same is true for Eurasian Blackbirds and quite a few other songbirds for which we have data.
So one theme here is that of variation. Variation and complexity are recurring themes in nature. This keeps things exciting and not entirely predictable.
Migration paths and pacing in birds often show variation across individuals, sexes, age classes, different breeding populations, and even subsets of those populations.
Now, I’d love to get into the many ways that biologists track birds on their migrations. But that’s a big topic in itself and probably warrants a whole separate article.
For now, just know that there’s a handful of methods that have been used, from simple leg bands to fancy GPS tracking. Recent advances in technology have truly revolutionized the way scientists record the movements of birds.
Here is one fun story, though, related to tracking:
In 2016, researchers in China placed small satellite tracking tags on a few Common Cuckoos. They wanted to figure out where Chinese populations of this species spend the non-breeding (i.e. winter) season. School kids were tasked with naming the tagged birds. The two most famous cuckoos were the ones with the most marvelously awesome names: Flappy McFlapperson and Skybomb Bolt. The scientists and cheering public followed the progress of these birds, in near real-time thanks to the satellite tags, as they flew south to India, across the Indian Ocean, and all the way down to southeastern Africa. You go, Flappy and Skybomb!
Until this study, nobody knew that these cuckoos flew all the way from China to Africa every winter.
One last point about this story is that Common Cuckoos are brood parasites. They are raised in the nests of other bird species. This means that Flappy McFlapperson didn’t have any parents to show her the way to her non-breeding range. She had only her instincts for guidance on her first journey. How crazy is that?
Concentration Points, Staging Areas, and Stopovers
Now let’s talk about some important geographic features related to migration.
Let’s consider those intrepid, long-distance migrants. Members of such species are often migrating in flocks, sometimes by the many thousands. In certain places, these birds come to concentration points as they fly along their migration routes, where they’re funneled into narrow places. These birds—of one species or perhaps many species—are passing through the same geographic bottleneck.
Concentration points exist because some landscape features and/or weather patterns force birds to take particular routes—routes that offer the least resistance.
Famous examples of concentration points include Hawk Ridge in Minnesota, the Straits of Gibraltar in Spain, and Centro de Veracruz in Mexico, home of the world-famous “River of Raptors.”
Concentration points are exciting places for us birders because they’re places we get to see tons of birds, possibly of many species, in a small geographic area and over a short time period.
So that’s concentration points.
Then we have staging areas. These are places where birds stop on their long migrations, where they will hang out for a relatively long period, sometimes for weeks. Staging areas are where birds are fattening up on some local food resource before they hit the skies for a long, arduous leg of their journey.
These areas are kind of like gas stations or fast-food restaurants for birds, but instead of just making a quick stop, they just keep ordering more greasy cheeseburgers, for days and days on end, before they finally get back on the road.
During their spring migration northward along the Atlantic coast of the US, Red Knots stop on beaches in New Jersey and Delaware to feast on the eggs of Horseshoe Crabs. During the 11-12 days a Red Knot will spend at this staging area, it will probably double its body weight by eating the fat-rich eggs.
Besides staging areas, there are stopovers. Stopovers are where birds will land for relatively short durations to rest and to do some feeding. Migrants might spend one to a few days at a stopover. Recall the Swainson’s Thrush I mentioned earlier, which makes many ‘hops’ during migrations, hanging out for a few days at each stopover.
Migration Flyways and Broad-front Migration
Now, I know I made a point of talking about individual variation in migration paths earlier. But it’s still true that within a species, individual birds follow fairly predictable routes. And often multiple species will follow similar routes, passing through the same geographic regions on their migrations.
These multi-species shared routes are what we call migration flyways. You’ve probably seen these depicted on maps. There are specific flyways for various regions of the world. These are routes that multiple species take at an intercontinental scale.
Where I live in Oregon in the US, migrating birds are in the Pacific Americas Flyway. This is one of eight named flyways. Others include the Central Asia and the East Atlantic flyways.
Flyways are a classical way to envision the migration routes of birds. As usual, reality is messier than our tidy maps of avian flyways suggest. But, if you can ignore a bit of individual and interspecies variation, flyways are still a meaningful way to think of migration.
In contrast to the relatively narrow routes of flyways, some species migrate in what we call broad front migration. This is where members of a species just fan out across a continent as they’re all heading south or north. They aren’t restricted to one narrow band or flyway.
How Do Birds Know When to Migrate?
Now, imagine you’re a bird, and spring is approaching. It’s almost time to take off for your breeding site, thousands of miles away.
But how do you know when to take off, to start your very long and perilous journey? You can’t read, not even at a 3rd grade level, and you don’t own a smartphone, so looking at a calendar app isn’t an option.
Well, you, as a migratory bird, notice that you’re feeling all fidgety and anxious this spring. And now that you think about it, you feel this way every year. But there’s nothing in the environment that you’re consciously aware of that makes you feel this way. You just feel like you gotta move. You gotta get the heck out of here!
This overwhelming restlessness you feel in your bones has a name: Zugunruhe.
Zugunruhe is the technical and very German word for the increased anxious behavior that migratory birds show when it’s time for them to migrate. In many species, this behavior is strongest at night, which is when these birds normally fly during migration.
Migratory birds are programmed by their genes to show this behavior, because they have an internal biological clock. This clock is driven by genetically determined hormonal changes that happen on an annual cycle.
The clock is calibrated by photoperiod. When birds experience normal patterns of natural daylight, their clock is calibrated on a 12-month cycle. Under experimental conditions, however, the clock of a bird not allowed to experience its normal photoperiod will drift a bit, so that maybe its hormones end up on a 10-month cycle or a 14-month cycle. With a normal photoperiod, the clock gets calibrated correctly, and all is well.
So there is an instinct to migrate at a particular time of year, an instinct driven by genes. And these genes have been honed by evolution.
Researchers have conducted many studies over decades to figure all of this out. And even more migration research has focused on what we’re about to talk about: orientation and navigation.
Orientation and Navigation During Migration
After a bird gives in to its inescapable impulse to migrate, it still has the problem of figuring out where to go and how to get there.
To solve this problem of orientation, our bird needs three things: a compass, a map, and a way to tell time.
As for telling time, I already mentioned the biological clock that birds have for their annual cycles. Birds also have an innate clock that tells them the time of day. This, too, must be calibrated by the rising and setting of the sun.
Knowing the time allows birds to use other signals, such as the position of the sun and stars, to determine direction.
But many, if not most, birds appear to be born with the ability to sense direction. Sensing direction by using the Earth’s magnetic field is generally thought to be the most important orientation tool for birds. This is their compass. But there is plenty of evidence that at least some birds also use the sun and stars to determine direction.
So we have a way to tell time and a compass. But some sort of map is needed or successful migration.
In some species, such as many geese and other waterfowl, juvenile birds in their first year follow adults to their non-breeding areas. These young birds note landscape features such as rivers and coastlines so they can make the journey on their own the next time. They may also incorporate the patterns of constellations in the night sky. They end up with a mental map of their migration route.
Many other bird species don’t have the luxury of being guided by their parents on their first migration. Instead, these species are actually born with the basic directions they need hard-wired into their brains. Their instincts tell them which direction to go, when to go, and how long they should fly before stopping.
This is just amazing to me. Natural selection has programmed these birds to fly, sometimes thousands of miles, to a place they’ve never been. Just like Flappy McFlapperson, they end up where they need to be, or close enough to it, to breed successfully and survive.
Not surprisingly though, there is still plenty of learning that takes place. Young birds are more likely to get off course and end up in the wrong place. Veteran migrants have developed much more detailed mental maps and end up exactly where they need to be with much more regularity. They integrate the magnetic field with visual landmarks and possibly even smells and sounds to create an accurate map.
So much more could be said about how birds find their way around the world. For now, though, let’s move on to the physical challenges and changes that birds go through during migration.
Bird Physiology During Migration
For a small animal like a bird, undertaking these journeys is risky and energy demanding.
Migration takes a lot of energy, because these animals are flapping their wings for many hours—sometimes days—without stopping.
To prepare for their migrations, birds need to store up a bunch of energy. Fat is the best nutrient for energy storage. For its weight, it has the most for bang for the buck. Birds preparing for a long haul will gorge themselves on fat-rich insects, mollusks, nuts, or berries.
This behavior of eating like crazy is called hyperphagia. This is the same thing animals like bears do before going into hibernation and what humans do for no good reason during the holidays.
When birds are feeding and getting ready to migrate, or when they’re at staging areas, their digestive systems grow dramatically, to allow them to eat enormous amounts of food. They pack on lots of fat, relatively quickly. When it’s time to fly and to migrate, their digestive systems contract and shrivel up since these organs would just be dead weight while the bird is flying.
Conversely, their pectoral and other flight-related muscles grow and their respiratory systems expand to become powerful and efficient.
Flying fast and furious over long distances would be hard enough, but birds also have to deal with challenges along the way, primarily wind and storms. Flying into a headwind—wind that’s coming at you head-on—is going to increase the number of calories you need to burn per hour.
So birds prefer a tail wind and the timings and the paths of their migrations often take advantage of tail winds and mild weather.
Some types of birds migrate during the day, others do so at night. Large soaring birds like raptors commonly migrate during the day, because their broad wings allow them to make use of thermals and updrafts to conserve energy. Smaller birds can’t really do that. Smaller species often migrate at night so they can avoid aerial predators like falcons and perhaps take advantage of cooler temperatures and calmer atmospheric conditions.
Migrating birds often become exhausted when they’ve been flying nonstop, and even more so if they run into a storm or strong headwinds after a long flight. These birds will sometimes “fall out.” This is an actual technical term in ornithology: fallout. When birds hit poor weather after a long journey, often after flying over the ocean or other large body of water, they will abruptly come down to land to rest and recover. Sometimes this happens all at once to thousands or even millions of birds, across multiple species. Like the concentration points I mentioned earlier, fallouts can be very exciting to birdwatchers, because we can see lots of migrant birds, all resting in the same area.
The migrations of birds connect all the planet’s continents in a way that no other biological process can match. Birds show us how interconnected our world is and how important it is that we protect natural habitats, not just in our own backyards, but everywhere.
My wife and I are so happy when the Swainson’s Thrushes return to our yard every May. Their upward-spiraling song is a defining sound of our verdant spring. We love knowing that these thrushes are breeding in our forest and that we can offer them sanctuary.
But those same birds might fly all the way to Peru in autumn. Their survival depends just as much on the preservation of their habitat in that country.
By understanding the migratory patterns and behaviors of birds, we are better equipped to help them. Bird migration is one of the great wonders of the natural world, and we should do what we can to keep the flyways filled with rivers of migrating birds.