Tropical birds migrate without riding the wind

Every year, billions of birds take on epic journeys between North America’s temperate zones and the tropical forests of South America.

While their migration patterns have been studied extensively, most of what we know comes from data collected in North America and Europe. That’s left a big gap in our understanding – especially in tropical regions.

Researchers from the University of Chicago, working with scientists in Colombia and at universities across the United States, are now filling in those gaps.

The team used radar data from across Colombia to see how birds behave when migrating through tropical skies. What they found is changing how we think about migration, weather, and conservation.

Birds migrate without weather cues

In places like the U.S. and Canada, bird migration often happens in big waves. Birds wait for just the right wind conditions – usually during cold or warm fronts – then take off by the thousands in a single night. But in Colombia, birds follow a different rhythm.

“Birds are great indicator species of environmental health, serving as bellwethers of global biodiversity gain and loss,” said Jacob Drucker, a doctoral student at the University of Chicago. “So understanding their migration on a global scale is extremely important.”

“But until now, our knowledge of this massive natural phenomenon has been heavily biased towards temperate latitudes in North America and Europe.”

Unlike temperate zones, Colombia doesn’t have strong seasonal wind patterns. Instead, its weather is more stable. The study revealed that birds migrating over the country travel at a steadier pace. They don’t wait for perfect nights – they go when they need to.

“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 they fly,” noted Drucker.

Soaring to beat the wind

The Colombian Andes offer a special case. High above the forests, winds blow southward as part of the Orinoco Low-Level Jet.

These winds can either help or hinder birds, depending on the season. In the fall, they act as tailwinds. In the spring, they become headwinds.

“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.”

These shifts in altitude helped them conserve energy as they responded to changing wind conditions in the Andes.

Migrating birds tracked by radar

This study wouldn’t have been possible without an unusual data source: Colombia’s national weather radar network. Seven years ago,

Drucker reached out to fellow researchers with a simple idea – what if they used weather radar to track birds? He teamed up with Nick Bayly from the Colombian conservation nonprofit SELVA and Alfonso Ladino, a meteorologist with ties to Colombia’s weather agency and the University of Illinois Urbana-Champaign.

Together, the researchers began a multi-year project to decode the tropical migration story. A major challenge was filtering out radar ‘noise’. In tropical skies, there are a lot of insects – more than in temperate regions – and they can look a lot like birds on radar.

“We initially struggled to distinguish between insects and birds,” said Drucker. “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.”

The researchers tested that model using radar data from both Colombia and Australia. It’s likely to help other scientists in future studies.

Bird-safe cities need rethinking

The study doesn’t just add to our understanding of bird behavior – it could shape how we protect birds in cities.

In cities like Chicago or Toronto, turning off lights during major migration nights can reduce bird deaths caused by building collisions. These “lights out” campaigns are carefully timed based on weather forecasts and migration patterns. But that strategy might not work in tropical cities.

“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.

That means new strategies will be needed in cities like Bogotá, Medellín, and Calí. Conservation efforts in the tropics may need to focus less on timing and more on continuous solutions.

The next migration frontier

This study is just the start. There’s still a lot to learn about how birds react to local weather and terrain as they move. For Drucker and his team, the goal is to go deeper – zooming in on finer scales to understand every twist and turn in a bird’s journey.

“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.

By combining new technology with international teamwork, researchers are opening new ways to better understand and protect migratory birds – wherever they fly.

The full study was published in the journal Proceedings of the Royal Society B Biological Sciences.

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