Scientists have long surmised that birds migrate during winters to save energy. The reasoning has been pretty obvious: far away from the biting cold, birds would need to expend less energy to keep themselves warm.

New research has upended that assumption.

A study published in the journal Nature Ecology & Evolution found that one group of Eurasian blackbirds (Turdus merula), a partially migratory species, spending time in warmer regions didn’t save energy when compared to a population of the same species that stayed put in a frigid environment.

“We suspected an energy deficit for the resident birds but there was no significant difference between the net energy they spent compared to the migrants,” Nils Linek, lead author and researcher at the Max Planck Institute of Animal Behavior, told Mongabay in a video interview. “If there’s no energy benefit overall, there must be other cues or reasons to migrate.”

The research is potentially the first to measure and track physiological conditions of birds over the course of their winter migration. Similar studies usually take place in a lab setting. “There are so many textbook theories about migration but until now, we haven’t been able to test this in the wild,” Linek said.

As part of the study, the researchers deployed biologgers that were surgically implanted into the birds. The sensors in the loggers measured heart rate and body temperature every 30 minutes. Heart rate serves as a proxy for energy expenditure; the more the energy used, the higher the heart rate. “It was like peeking through a keyhole into their internal systems like they are wearing a smartwatch,” Linek said.

Over the course of three years, starting in 2016, the team deployed sensors on 118 birds in southern Germany. Externally attached radio transmitters enabled the scientists to monitor if the birds were dead or alive, as well as to track their movements. In the end, the team had data amounting to more than 890,000 measurements from the loggers they retrieved from 83 birds.

“We ended up with a novel picture of what these birds are doing in order to support their migration,” Scott Yanco, co-lead author and research fellow at the University of Michigan’s School of Environment and Sustainability, told Mongabay in a video interview. (Yanco was affiliated with Yale University while he was part of the research.) “They are tuning their heart rates and their entire metabolic organization in really complicated ways,” he said.

Analyzing the data on heart rate and body temperature helped the scientists calculate the daily energy expenditure of the birds. They found that migrating birds started lowering their heart rate and body temperature almost a month ahead of their departure to save energy for the journey. Theoretically, their energy expenditure is supposed to go down once they reach their destination. But this wasn’t found to happen. “It doesn’t show up in the heart rate data,” Yanco said. “There’s energy that’s running through their system that’s unaccounted for.”

The findings have left the team with some intriguing questions: Where did this energy go? And if there’s no energy benefit to migrating, why do so many birds do it?

Linek and Yanco hypothesize that there must be hidden costs to migration that could potentially include immunological functions, the energy required to remain vigilant in a new environment, or competition from other birds in the area they’ve migrated to. “The thermoregulatory benefit is clearly offset by something else that is harder down there,” Yanco said.

While figuring out answers to these questions, Linek said, he wants to tap into the “revolution in biologgers” to gather more data and better understand how the environment “is traveling through the physiology and energetic organization of birds.” Understanding physiological responses to migration will also help scientists predict how climate change and habitat degradation might impact migration patterns in the future.

“Predicting where and when animals are going to be, what causes their populations to go up or down, and how we can intervene to change those things,” Yanco said. “Those are the questions we need to answer.”

Banner image: New research has found that migrating Eurasian blackbirds didn’t save more energy compared to members of the same species that stayed behind. Image by Christian Ziegler/Max Planck Institute of Animal Behavior.

Abhishyant Kidangoor is a staff writer at Mongabay. Find him on 𝕏 @AbhishyantPK.

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Citation:

Linek, N., Yanco, S. W., Volkmer, T., Zuñiga, D., Wikelski, M., & Partecke, J. (2024). Migratory lifestyle carries no added overall energy cost in a partial migratory songbird. Nature Ecology & Evolution. doi:10.1038/s41559-024-02545-y