Study of four crane species reveals complex relationships between birds and their environment

Understanding how animals use their environment to survive and thrive is a key challenge for predicting the impact of climate change on wildlife. A global collaborative study of four crane species has shed light on how migrations are finely adapted to unpredictable and complex environments.

A team from 10 countries combined new animal tracking technology, remote sensing information about the environment and a new statistical framework to better understand four iconic species: common cranes, white-naped cranes, black-naped cranes and demoiselle cranes.

The study, conducted by scientists from the Max Planck Institute for Animal Behavior and Yale University, was published in Proceedings of the National Academy of Sciences September 23.

Researchers used tiny GPS tracking devices to track the movements of 104 cranes in Africa, Asia and Europe. These devices included unique solar-powered GPS bracelets developed by MPI-AB scientists. The tracking data revealed the impressive migrations undertaken by the cranes.

Some migration routes exceeded 6,400 km round trip and required crossing barriers such as the Alps or the Himalayas, the deserts of the Arabian Peninsula or the Mediterranean Sea.

In addition to the tracking study, the researchers also developed a statistical framework that revealed how crane movements are related to aspects of the environment, such as the presence of nearby crops or bodies of water, as well as the temperature and vegetation cover of the land.

“Animals have to meet their own needs with what they can get from their environment, but both of those things are constantly changing,” said Scott Yanco, first author of the study and a postdoctoral researcher at the University of Michigan.

“This creates an intriguing optimization problem that we wanted to see if cranes solved through long-distance migration.”

The researchers found that the four crane species experienced very different environmental conditions over the course of a year, and that these periods were synchronized with important events in their lives. This was particularly evident when comparing temperatures or resource availability at wintering and summer breeding sites.

For some species, the migrations themselves brought about huge changes in environmental conditions. For example, demoiselle cranes crossed the Tibetan Plateau and had to cope with major temperature fluctuations.

“We think this is all related to different biological needs at different times of the year,” adds Yanco, who conducted the research while at Yale’s Center for Biodiversity and Global Change. For example, common cranes clearly favor agricultural areas in late summer, a time when they are rearing juveniles and preparing for fall migration.

“That’s exactly when we expect them to want easy access to food,” he says.

For other species, access to food can come at a cost. The black-necked cranes studied had to choose between safe habitat and abundant resources.

“Surprisingly, the balance between these competing needs changed over the year depending on what the birds were doing,” Yanco adds. During migration, they opted for safer resting conditions, while during breeding, they tended to favor abundant food.

“This kind of shift in emphasis depending on the cranes’ needs at a given moment is what we expected,” says Ivan Pokrovsky, a postdoctoral researcher at MPI-AB and the study’s last author.

“But we were impressed by how the cranes were able to use motion to resolve trade-offs between competing needs and to access certain environments during key times of the year.”

Understanding how animals interact with their environment not only gives us a more nuanced view of how they survive in complex environments, but is also essential for developing policy and management measures to address the twin crises of climate change and biodiversity loss, the authors say.

The study framework provides a statistical tool for understanding the complex relationships between animals and their environment that can be widely applied to wildlife conservation and management efforts.

“When we know how animals use certain environmental conditions, we can make better predictions about how species might respond to human-induced global change and develop more effective interventions that ensure the conditions species need to survive are preserved,” Pokrovsky says.