As spring unfolds in North America, the days grow longer and warmer, flowers sprout from thawing soil, and green leaves bud on branches. Bees are busy and birds are courting mates, building nests and raising young. As any hardy birder knows, many of these birds have been absent, spending the harsh, cold northern months in warmer southern climates. Some have flown thousands of kilometers to arrive at their breeding grounds at the same location around the same time as last spring and the spring before.
Why birds undertake these seasonal journeys seems intuitive at first: they want to improve their chances of survival and those of their offspring. But how bird migration originally evolved has actually been a topic that scientists have debated for a long time.
Researchers don’t know enough about ancient birds and their closest relatives many millions of years ago to determine when the first avian ancestors began migrating. Existing evidence goes back at least hundreds of thousands of years. For example, genetic studies suggest that Galapagos hawks separated from Swainson’s hawks between 125,000 and 250,000 years ago, when Swainson’s hawks may have been distracted from their migratory journey. However, a deeper chronological tracking of bird migration has not been possible so far. In reality, the bird migration could be much older.
“When we talk about the evolution of migration in modern times, we’re actually talking about the modification of this ancient trait, or probably a set of traits,” says Robert Zink, an avian evolutionary biologist at the University of Nebraska. A number of creatures like squid, butterflies and reindeer have evolved migratory behaviors, says Zink, and this suggests migration is a fundamental response to take advantage of seasonal resources. In other words, it might be more surprising if birds didn’t migrate with their flight powers.
For decades, debate over how the first birds began migrating revolved around the southern versus northern home hypotheses. Essentially, these ideas suggest that modern migratory species originally lived in either tropical (a southern homeland) or temperate regions (a northern homeland) and gradually evolved to travel between the two climates to exploit food, competition to escape or breed in places with comparatively fewer predators. Whichever direction the migrants came from, birds with access to better resources survived and reproduced more successfully, passing on the urge to travel to offspring.
More recently, some researchers have viewed migration somewhat differently. By tracing evolutionary relationships between species and examining modern breeding biology, ornithologist Benjamin Winger theorizes that migration is closely related to the seasons — regardless of the geography or latitude a species originates from.
“The fact is, birds are everywhere, adapting to their current conditions,” says Winger, who studies bird migration and dispersal at the University of Michigan. From this perspective, birds migrate to escape dangerous environments that threaten survival—not much different than hibernation in bears or diapause in insects. “To live in these high latitudes, [birds] must be fundamentally migratory,” he says. (Evolution always provides exceptions, however. Winter burrowing grouse are among the few birds that live year-round in high northern latitudes).
As such, flying south to escape winter when food is scarce is just one example of why birds migrate. The theory also extends to birds migrating up and down mountains or in tropical areas to follow seasonal rainfall, temperature, and other environmental patterns.
Of course, escaping difficult conditions can’t be the whole migration story: “The tricky part was explaining why they’re coming back next year,” says Winger. He says that birds – like many organisms – prefer to be in the places where they were born. “Basically, what drives them back is the benefit of returning to a familiar breeding ground,” he says.
A bird’s migration distance may also be related to the reproductive strategy it has developed. According to Winger’s work, a long-distance migrant such as the Swainson thrush produces fewer offspring than the hermit thrush, which does not travel as far. Surprisingly, despite their arduous journey, adult Swainson’s thrushes have better survival rates. Similar evolutionary tradeoffs between fewer offspring surviving longer, like an elephant, or more offspring dying earlier, like a mouse, can be seen across the animal kingdom. This data shows that long-distance migrants may resemble an elephant more than a mouse. “Migration essentially promotes this strategy with higher survival and lower reproductive performance,” says Winger.
However migration first evolved, the drive for modern birds to migrate is at least partly genetic, says Kristen Ruegg, a researcher at Colorado State University and leader of the Bird Genoscape Project. For example, in crossing experiments, scientists breed a bird (like a blackcap) that flies a long distance in one direction with another bird (like a blackcap from a different population) that flies a short distance in a different direction. Consequently, the offspring will attempt to migrate an intermediate distance and in an intermediate direction.
These results indicate that migration patterns can be passed or modified across generations. But understanding which specific genes are responsible was “much harder than you might think,” says Ruegg.
Bird researchers are also trying to better understand how genes code for migration timing. All animals have internal clocks that react to environmental stimuli such as light, temperature or stress. For example, these clocks are responsible for the circadian rhythms that humans experience in the 24-hour cycle. In birds, deciphering the genetic material that controls these clocks could shed light on how and why migration evolved, Ruegg says.
Other genetic questions remain. For example, not all birds are migratory. Scientists believe these species still have all of those genes, but that they’re just turned off. But that’s not certain, says Winger.
How quickly migratory traits can evolve over generations is another important question in this era of rapid climate and environmental change. The fact that breeding birds in North America have adapted to glacial advances and retreats over the past million years suggests that migration behavior is flexible, says Zink. Researchers have even documented blackcaps and house finches developing new migration patterns within a few generations. But that doesn’t mean that all birds can or will adapt so fast
Much of this research is academic and nuanced. And with millions of birds migrating this spring, their feats may seem nigh impossible to casual observers looking at them. But it’s helpful to remember: For the birds, it’s not that remarkable — it’s just how they evolved to survive.