For migrating birds, fall brings difficulty and danger. To reach warm winter climes, many birds must fly hundreds or thousands of miles, expend immense amounts of energy and successfully dodge storms, skyscrapers and other potential threats.
Still, scientists have long assumed that a basic trade-off made migration worth the gamble: Once birds arrived at their wintering grounds, they wouldn’t need to work so hard to stay warm, saving substantial amounts of energy. “But nobody ever tested this,” said Nils Linek, a behavioral ecologist at the Max Planck Institute of Animal Behavior in Germany.
Now, Dr. Linek and his colleagues have done so. Their findings, based on a partially migratory population of German blackbirds, challenge the conventional wisdom. Even in the depths of winter, blackbirds basking in balmy southern Europe or northern Africa did not spend any less energy than those riding out the cold in Germany, the scientists found.
“It’s sort of shocking that there isn’t this net benefit,” said Scott Yanco, an animal ecologist at the University of Michigan and an author of the paper. (Dr. Yanco conducted the research while at Yale University.)
The study, which was published in Nature Ecology & Evolution on Wednesday, also revealed that the migrants began preparing for their fall journeys several weeks in advance, saving up energy for the flight by slowing their metabolism at night.
Together, the results suggest that migration is “way more complex than all the theories predicted,” Dr. Linek said. “There were a lot of surprises.”
The researchers studied common blackbirds that spend their summers in the forests of southern Germany. Most of the birds stay put for the winter, but about a quarter of them migrate, flying south in October and November. These migrants spend the winter in southern Europe or northern Africa, returning to Germany by early April.
Until recently, it would have been exceedingly difficult to track the energy expenditures of these small, wild birds throughout the entirety of their migratory cycle. But tiny, implantable data loggers now make that possible. “It’s basically like birds wearing fitness smartwatches,” Dr. Linek said.
These devices recorded each bird’s body temperature and heart rate every 30 minutes over the course of roughly nine months, from September through May. Heart rate is a well-known proxy for energy expenditure; the more energy a bird is using, the higher its heart rate. The scientists implanted the data loggers in 118 blackbirds and were ultimately able to recapture 83 of those birds to download the resulting data.
The data confirmed that the migration process itself, a journey averaging roughly 500 miles, required the birds to expend extra energy. And it suggested that the migrants began saving up for this cost in advance. Four weeks before departing Germany, the migrants began lowering their nighttime heart rate. About a week and a half later, their overnight body temperatures dropped, too.
“It looks like they’re turning down their thermostat,” Dr. Linek said.
These physiological shifts, which did not occur in the resident birds, helped the migrants conserve energy before their southward flights.
But during the winter, the migrants did not have lower heart rates than those that remained in frigid Germany. And over the course of the entire migratory cycle, both groups of birds ended up expending the same amount of energy, the scientists found.
The warmer weather did come with benefits. Using computational modeling, the researchers calculated how much energy it would be needed for each bird to maintain its body temperature given the local weather conditions. The migrants maintained slightly higher body temperatures than the German residents throughout the winter — and they expended a lot less energy doing so.
Over the course of the winter, the researchers found, a blackbird that remained in Germany needed to spend an extra 18,600 kilojoules — the equivalent of roughly 4,400 calories — just trying to stay warm. That’s a significant difference for a bird that weighs just a few ounces, Dr. Yanco said. “But then in the heart rates, it doesn’t show up,” he added.
That means that whatever energy the migrants were saving on thermoregulation was being spent on something else. “We don’t totally know where that energy is going,” Dr. Yanco said. “That’s sort of the next big mystery to solve.”
The journey itself cannot account for all that energy, but there could be other “hidden costs” associated with migration, Dr. Linek said. For instance, maybe migrants spending the winter in unfamiliar territory must be extra vigilant about threats.
Another possibility, still speculative, is that migrants can invest that extra energy in some other process, such as reproduction. That might allow migrants to produce slightly bigger eggs or provide more parental care, giving them a selective advantage even without any net energy savings.
Although many questions remain, new technologies, like the miniature data loggers, are presenting scientists with opportunities to ask and answer questions that were once inconceivable, Dr. Yanco said. “The rapid advance in what we’re able to measure on free living animals is exponential, year over year,” he said. “We’re really in this exciting discovery phase.”
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