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Tropical bird migration shaped by stable weather and shifting wind altitudes, study shows

Every year, billions of birds undertake intrepid journeys between temperate regions in North America and their tropical wintering grounds in South America.

"Birds are great indicator species of environmental health, serving as bellwethers of global biodiversity gain and loss, so understanding their migration on a global scale is extremely important," said Jacob Drucker, a doctoral student at the University of Chicago. "But until now, our knowledge of this massive natural phenomenon has been heavily biased toward temperate latitudes in North America and Europe."

Drucker is the lead author of a first-of-its-kind international study titled "," that leveraged weather radar networks across Colombia to examine how migrating birds respond to different atmospheric conditions in the tropics compared to those in northern temperate zones.

Their findings, which appear in Proceedings of the Royal Society B: Biological Sciences, reveal how the relatively stable weather conditions of tropical climates shape bird migration in distinct ways that expand ecological understanding and could impact future conservation efforts.

How tropical winds change migration strategies

In temperate regions, predictable cycles of warm fronts and cold fronts make some nights much more favorable than others for the flight of migratory birds.

As a result, bird migration in these areas typically happens in dramatic bursts, with huge numbers moving simultaneously during optimal wind conditions. In Colombia, those predictable cycles don't really exist; as a result, the researchers observed, birds migrate at a steadier, more gradual pace in tropical conditions.

"It turns out that wind isn't as important to birds in deciding which nights to fly in Colombia, but it plays a major role in deciding the altitude at which they fly," Drucker said.

In the Colombian Andes, birds encounter consistent winds blowing southward at varying altitudes, known as the Orinoco Low-level Jet. These winds act as helpful tailwinds for birds traveling south in the fall, but become headwinds in the spring. To conserve energy, birds adjust how high they fly to avoid the altitudes where the wind is blowing most strongly at any given time.

"We saw birds flying as high as 3,000 meters above the Amazon to avoid headwinds. It was spectacular," Drucker said. "When the wind weakened at lower altitudes, birds adjusted by flying lower."

International collaboration and innovative methods

This research was made possible through international collaboration with Colombian scientists and organizations, as well as experts from universities across the U.S. and Chicago's Field Museum.

Seven years ago, Drucker approached several colleagues about using relatively new radar data from the , Colombia's national weather service. Together with Nick Bayly, director of migratory bird studies at the Colombian non-profit SELVA, and Alfonso Ladino, a meteorologist at the University of Champaign Urbana who previously worked with the IDEAM, he began building a project and partnership that would lay the groundwork for unprecedented analysis of bird migration.

Other collaborators at Cornell University, including Drucker's mentor Adriaan Dokter, Ph.D., played a critical role in developing advanced analytical methods to separate flocks of migrating birds from swarms of airborne insects—a significant methodological challenge in tropical environments.

"We initially struggled to distinguish between insects and birds," Drucker recalled. "There are far more insects in the tropics, creating substantial radar noise from an ornithological perspective. To solve this, we built a model that assumes insects move passively with the wind, while birds move independently and purposefully."

This approach, using radar data from both Colombia and Australia, will also likely prove invaluable for future ecological research.

Conservation challenges in tropical cities

Understanding migration dynamics in Colombia has real-world conservation implications. Cities pose significant risks to migratory birds, especially through collisions with the windows of brightly-lit buildings at night.

In temperate regions, coordinated "lights out" campaigns timed with predictable large-scale migration events can help protect birds, but the new findings indicate that these approaches may face challenges in tropical cities like Bogotá, Medellín and Calí.

"Since bird migration in Colombia isn't tied to predictable wind events, it becomes much harder to anticipate large migration nights and tell people when to turn their lights off," Drucker said. To effectively implement conservation measures, ecologists and policymakers will need to develop new strategies.

Looking ahead, Drucker emphasized the need for continued research to understand migration at even smaller scales.

"We need to drill down into the granular details of how birds react to specific habitats and finer-scale weather patterns along migration routes," he said.

As scientists expand radar coverage and refine their methods, they are opening new avenues to protect migratory birds in a rapidly changing world.