Why Do Birds Migrate? Scientists Have A Few Major Theories.

At one point in the not-too-distant past, winter’s lack of birds completely flummoxed some of history’s greatest thinkers. Just a few hundred years ago philosophers posited that birds buried themselves in swamps, flew to the moon, or fully transformed into other animals during the cold months. Thanks to dedicated scientific observation, we now know that birds are neither interstellar travelers nor capable of full-body alchemy.

Twice a year, across the globe, billions of birds take flight on (Earth-bound) migrations. From hundreds to thousands of miles in length, these seasonal journeys keep the beat of the natural world–heralding annual cycles and transporting nutrients and biodiversity far and wide. But why do birds go through all that work? 

The answer is harder to parse out than you might expect, with conflicting theories and limited, tough-to-collect data to go off of. Recent research highlights that there are still serious surprises to be uncovered. For decades, many ornithologists have assumed that migration must ultimately save birds effort, offering some clear energetic benefit over the course of a year. However, in a recent study tracking European blackbirds’ heart rates and body temperatures over months, scientists have found that the total energy expended by birds that stay in place through winter and those that migrate to warmer regions is about equal. These findings, published September 18 in the journal Nature Ecology & Evolution, add to the migratory mystery. If there’s no outright energy savings to avoiding winter, why fly so far just to turn around a few months later?

What we know

In one sense, explaining migration is really simple, says Scott Yanco, an animal ecologist at the University of Michigan and a co-author on the September study. “It’s a response to the flux of seasonality on Earth,” he says. Migration happens because seasons change, which shifts the resources available and day-to-day conditions in a place. 

But go one level deeper and the answer to why birds migrate becomes murkier, Yanco adds. “That central question actually motivates a lot of my research,” he says. 

“It’s not the only way to deal with seasonality. There’s all kinds of other strategies that we see animals use to adapt,” Yanco tells Popular Science. For instance, there’s hibernation, metabolic torpor, brumation, seasonally-altered body composition, winter “coats”, or dietary changes. Not all birds migrate. Some deploy alternate strategies to make it through winter or nab the best breeding sites. Even amid a single species and population, a subset of individual birds may be migratory while others are year-round residents, as is the case with the partially migratory blackbirds in Yanco’s recent research. Given that, the question really becomes “why migration over the alternative options?”

Migration is difficult and dangerous. Long-distance travel can mean a struggle to find resources, exhausting endurance flights, navigating natural disasters, colliding with human infrastructure, and an increased risk of being eaten. Compared with stationary periods, some research has found that birds have more than a four times higher risk of dying while migrating. Amid all of these challenges, the only sure thing is that the benefits of migration–like all traits and behaviors honed by evolution–must outweigh (or at least equal) the downsides. Otherwise, it wouldn’t have emerged in the first place, nor continue to persist generation to generation, says Yanco.

Conversely, sticking out the winter can also be dangerous, in some cases more so than migrating. One study of European blackbirds found that migrants had a significantly higher chance of surviving harsh winters than resident birds in the same population. As a trade-off, the study authors hypothesize that resident birds have a reproductive advantage, claiming the best territory before the migrants can get back. 

What we think

There are two prevailing and competing theories for how and why bird migration emerged. In the “temperate home” hypothesis, migratory birds started out living in climates with seasonally cold winters. Slowly, over time, they evolved to migrate to warmer zones in the winter to escape the harsh weather and the risks that come with it. The alternate “tropical home” framework suggests that migratory species started in the tropics and then evolved to leave during breeding season to avoid competition and take advantage of underexploited resources. 

A 2019 review study concluded that the former explanation fits better for most birds than the latter. Based on available data, migration seems to be more about escaping harsh winters than exploring new breeding territory. However, that prompts the next question: Why come back to temperate (or even Arctic) breeding grounds, if you can comfortably live in a tropical region?

A long-standing theoretical answer to that conundrum again comes down to competition. Maybe the crowds get to be too much when all the birds flock to the tropics. But another possibility is that there’s a specific benefit to coming home. That same 2019 review suggests that birds are innately driven to return to their familiar breeding grounds, where they know the dangers, where to find the best grub, and how to build a prime nest. 

Birds often display incredible site and resource fidelity, notes Yanco–returning not just to the same region, but to the same tree and nest perch year-to-year. “There’s a lot of memory and habit that goes into these things. It may be that that’s advantageous.” 

What’s still up in the air

Without firm data, which generally requires capturing, tracking, and re-capturing birds over multiple seasons, all of the above hypotheses remain educated guesses. It’s possible, Yanco says, that different species in different places are driven to migrate by different factors. “Understanding these processes is going to require understanding it for a lot of different taxa,” he says. And some research only ends up revealing just how much we still have left to learn. 

Nils Linek is the lead author of the European blackbird study published this month, and an evolutionary biologist and ecologist at the Max Planck Institute for Animal Behavior in Germany. He spent 11 months at a time in the field for three consecutive years capturing and monitoring blackbirds. In that time, he and his colleagues managed to implant 118 birds with pill-sized devices to monitor their heart rate and body temperature, as well as location transmitters. Linek, Yanco, and their co-authors were able to re-capture and analyze data from about 80 of those birds. 

They observed a couple of unforeseen results. First, migratory birds in their study displayed a never-before-seen period of metabolic deceleration in the month leading up to their departure. Their body temperatures and heart rates were significantly lower and slower than their counterparts who didn’t go on to migrate for a 28-day period. “It was very unexpected to see this [metabolic] preparation,” says Linek. “It’s a very well-hidden mechanism.”

Then, they found a surprising overall balance in the energy expended between migratory and resident birds. Based on temperature data for each location in the study, migratory birds would have had to expend far fewer resources on staying warm. Migration itself takes energy, and a corresponding spike in heart rate was visible in the data, but that increase only offset a small part of the thermoregulatory gain. In theory, the migratory birds should have been able to use far less energy overall by avoiding the cold. But still, by the time of their recapture, the migratory birds hadn’t saved any more energy than the ones who just stayed home, their hearts had beat an approximately equal number of times. “The equality of energy overall was mind blowing,” says Yanco. Somehow, the birds used up that excess energy, but how and where is unknown.

“We have ideas,” says Linek–but so far, no firm leads. The scientists suspect that the birds might be re-allocating their bodily efforts towards reproductive tissue or banking it for later as energy storage. Or it could be that life in the tropics isn’t a stress-free vacation after all–perhaps birds have to search farther and wider for suitable food or battle other birds to get the goods. Or maybe, when they’re in a warm place, birds simply move a lot more day to day than they do in the depths of winter. 

There are lots of questions remaining and lots of research left to do. “We’re just starting to pick up the puzzle pieces,” says Yanco. Yet the technology for studying birds and animal movements is the best it’s ever been and we’re getting closer to answers. With bluetooth, satellite tracking, and ever-smaller devices and implants, we have more ways to collect data than ever before. From theories of avian lunar landings to the present-day “we’ve come so far,” says Linek. Perhaps not as far as the birds do, but solving the mystery of migration may be our next destination.

This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.

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