Every year, countless birds migrate to avoid the cold winter weather, but a recent study by the Max Planck Institute of Animal Behavior (MPI-AB) reveals that staying in a warmer climate does not actually result in energy savings for these birds. Researchers utilized tiny loggers implanted in wild blackbirds to gather continuous heart rate and body temperature data every 30 minutes from autumn through the next spring. This has provided researchers with groundbreaking insights into the energy expenditures associated with migration compared to resident behavior, as well as uncovering a newly identified method that migrating birds use to conserve energy before their journey. The research was published on September 18 in the journal Nature Ecology & Evolution.
“We were surprised to find that migrating birds do not gain any overall energy benefit from escaping harsh winters,” explained Nils Linek, a co-author of the study and a researcher at MPI-AB. “It has long been a common belief that animals save energy by migrating to milder regions, but our results indicate that this is not the case. The reality of migration energy dynamics is much more intricate and intriguing than previously thought.”
Animal migration showcases how species adapt to seasonal changes, but the deeper question of why they migrate has remained difficult to resolve due to challenges in studying free-living animals over extended periods. The MPI-AB and Yale University researchers made a significant breakthrough by using sensors to monitor the energy spent by blackbirds throughout the entire migratory annual cycle and integrating this data with models for thermoregulation energy costs.
The sensor data revealed that migrating blackbirds significantly reduced their metabolism three weeks before migration, which likely outweighed the energy used during their actual migratory flights. “They effectively lower their internal metabolic rate, which helps them conserve energy for the upcoming journey,” added Linek. Notably, during their time in warmer winter habitats, these birds did not show a decrease in their total daily energy expenditure.
“This outcome was unexpected,” stated Scott Yanco, co-first author from the Yale Center for Biodiversity and Global Change. “Our energy models initially indicated that migration should lead to energy savings due to the significantly lower costs of maintaining warmth in a mild climate.”
So, what happens to this expected energy surplus for migrating birds? Linek suggests that hidden physiological adaptations or undisclosed costs may exist, which offset the thermal benefits they should enjoy. These could involve factors like maintaining alertness in unfamiliar surroundings, immune functions, or other unknown stressors that might negate the advantages of a warmer climate.
The research focused on blackbirds in southern Germany. Similar to various populations across Europe, the German blackbird population exhibits “partial migration,” meaning some birds migrate south for winter in warmer areas like Spain and France while others remain in colder breeding grounds. The researchers surgically equipped 120 wild birds with miniature heart rate and body temperature loggers that collected data every 30 minutes from September until May, when the devices were retrieved. They also tracked the migratory birds with radio transmitters that alerted researchers when the birds left Germany in September and returned in March and April the next year. The resulting analysis encompassed about 1 million data points to compare the heart rate and body temperatures between migrant and resident blackbirds.
“The physiological data provided us with an extraordinary understanding of how birds go through and experience migration—from the flights themselves, to their recovery afterward, and their activities during winter,” remarked Tamara Volkmer, a co-author and doctoral student at MPI-AB. “By recording in-depth energy measurements over time, we could uncover the less visible costs associated with their remarkable journeys.”
The findings of this study indicate that migration’s risks and challenges are not balanced out by energy savings in warmer climates, raising new questions regarding the evolutionary motivations for migration. “This could affect how we view migration and its mechanisms across various bird species,” noted Linek.
The implications of this research extend to predicting how different species might react to future climate conditions, the authors noted. Jesko Partecke, the senior author and group leader at MPI-AB with two decades of studying blackbird migration, remarked: “By understanding how migration physiology works, we can better predict which species might adapt, which may change their migratory patterns, and which may be at heightened risk as global temperatures continue to rise.”
