Scientists quantify energetic costs of free-flying songbird’s migratory lifestyle

Millions of birds migrate every year to escape winter, but spending time in a warmer climate does not save them energy, according to a study by the Max Planck Institute for Animal Behaviour (MPI-AB). Using miniaturised recorders implanted in wild blackbirds, the scientists recorded detailed measurements of the birds’ heart rate and body temperature every 30 minutes from autumn to the following spring. This is the first time that the physiology of free-ranging birds has been quantified continuously at this scale over the entire wintering period.

The data provide unprecedented insights into the true energy costs of migrants’ and residents’ strategies and reveal a previously unknown mechanism used by migrants to save energy before migration. The findings are published September 18 in the journal Nature Ecology and Evolution.

“We did not expect to find that birds did not gain any overall energetic benefit from escaping cold winters,” says Nils Linek, first author of the study and a researcher at MPI-AB. “It has long been assumed that animals expend less energy when migrating to warmer locations, but our results showed that these savings do not add up. On the contrary, the energetics of migration are much more complex and interesting than theory predicted.”

Animal migration is a spectacular example of how animals adapt to seasonal changes. Yet the fundamental question – why? – remains a scientific enigma because of the obstacles to studying the physiology of free-living animals over long periods of time.

In the new paper, researchers from MPI-AB and Yale University unlocked an important piece of this puzzle by deploying sensors that measured the energy expenditure of robins throughout the annual migration, then combining the physiological data with modeling to calculate the predicted energetic costs of thermoregulation.

Sensor data showed that migrating blackbirds conserved considerable energy in preparation for migration by lowering their metabolism three weeks before departure, potentially eclipsing the energetic costs of migratory flights.

“They actually turn down their internal thermostat, which allows them to save energy for the journey ahead,” Linek says. Yet when migrants are in warmer wintering areas, they don’t appear to decrease their total daily energy expenditure.

“This is not what we expected,” says Scott Yanco, co-lead author of the study from Yale’s Center for Biodiversity and Global Change. “The energy modeling we did for the study predicted that migration would likely create an energy surplus because of the greatly reduced cost of staying warm in milder climates.”

But where did this theoretical surplus of energy from migrants go? According to Linek, “We can only speculate at this point, but we think there may be other physiological adaptations or hidden costs that migratory robins face in their milder wintering grounds. These could be factors such as the need to maintain vigilance in new environments, immune functions, or unknown stressors that negate the thermal advantage they should have experienced.”

The team worked with blackbirds from southern Germany. Like many populations across Europe, German blackbird populations are “partly migratory,” meaning that some individuals migrate south to spend the winter in milder regions such as Spain and France, while others remain resident on the colder breeding grounds all year round.

The researchers surgically implanted miniature heart rate and body temperature recorders into 120 wild birds. The recorders recorded data every 30 minutes from September until the following May, when the devices were removed. The team also tracked the birds using radio transmitters, which signaled when the migrating birds left Germany in September and returned in March and April of the following year. The researchers analyzed the data from the recorders, totaling about 1 million data points, to compare differences in body temperature and heart rate between the migrating and resident robins.

“With the physiological data, we were able to observe with incredible precision how birds undertake and experience migration, from the migratory flight itself to how they recover afterwards and what they do during the winter,” says Tamara Volkmer, co-author of the study and a doctoral student at MPI-AB. “By recording detailed long-term energetic measurements of the migrants, we were able to glimpse the hidden costs of their impressive round-trip journey.”

The study’s findings suggest that the risks and challenges of migration are not outweighed by energy savings in warmer climates, opening new questions about the evolutionary drivers of migration more broadly. “This could have implications for our understanding of migration and its underlying mechanisms in different bird species,” Linek says.

The study also has implications for predicting how species might respond to future climate scenarios, the authors say. Jesko Partecke, lead author and group leader at MPI-AB who has studied robin migration for two decades, says: “Understanding the physiological underpinnings of migration means we can better predict which species are likely to adapt, which are likely to change their migratory patterns, and which are likely to face greater risks as the world continues to warm.”