In South America, Bird Migration Is Slow and Steady

From the Autumn 2025 issue of Living Bird magazine. Subscribe now.

For birders in the U.S. and Canada, fall migration means the chance of seeing massive numbers of birds on a single morning, or nearly none at all. That’s because birds migrating through North America will wait for just the right weather to fly en masse—a nice tailwind, or a calm night with no precipitation.

The longest-distance migrants need to make it all the way to South America, but they aren’t done when they pass the isthmus of Panama. Some, like the Blackburnian Warbler, migrate hundreds of miles farther through the Andes Mountains. Some head into the Amazon River basin, like the Scarlet Tanager.

According to a new study—the first-ever analysis of bird migration in South America using weather radar data—those warblers and tanagers are taking it easy once they hit the South American mainland. The research, published in June in the journal Proceedings of the Royal Society B, showed that birds migrating in Colombia move at a steady pace, rather than the stop-and-go rhythm of birds migrating through North America. 

“It’s just totally different from what we are used to in North America, where you have these alternations between really good migration nights, and then other nights where there’s almost nothing. You don’t really see that in the tropics,” says Adriaan Dokter, a coauthor of the study and a research associate at the Cornell Lab of Ornithology who specializes in radar ornithology. Dokter leads the BirdCast project, which uses weather radar data to monitor and predict bird migration in North America.

Previous studies using BirdCast in North America showed bird migration moving like traffic at a stoplight, where everybody stops or moves together in a race to take advantage of the green light. This new study depicts bird migration in South America moving like cars steadily flowing down an open road.

Colombian birding guide José Castaño says the study results align with his birding experiences in South America.

“Here in South America, we are habituated to observe the phenomenon of bird migration in a steady way,” says Castaño, who is also an ornithologist. He says one of his favorite birding moments every September “is when I can spot my first Blackburnian Warbler and hear the call of Summer Tanager in the shaded coffee plantation.” 

According to the study, the bulk of fall bird migration in North America takes place mostly on eight non-consecutive nights, creating pulses of migration in September and October when more than 50% of birds pass through. In Colombia, on the other hand, that same proportion of bird migration spreads out over 17 nights, as the birds move at a slower, steadier pace. 

“The steady pace … can be attributed to stable regional wind patterns [around the equator],” says Jacob Drucker, lead author of the study and a PhD student studying evolutionary biology at the University of Chicago. “The wind usually blows from the same directions during bird migration, as opposed to … temperate latitudes where wind direction changes every few days.” 

Drucker says that birds can fly whenever they want in South America because they can count on a consistent tailwind in the fall, unlike the fickle winds in North America that force birds to wait for a night with good weather.

The team of North and South American researchers began this study in 2018, when Drucker approached Alfonso Ladino-Rincon, a radar meteorologist and data analyst at the Colombian National Weather Service at the time, for weather-radar data. Ladino-Rincon is now a PhD student at the University of Illinois Urbana-Champaign studying climate, meteorology, and atmospheric science.

“The researchers came to me with their project, and I thought, ‘Wow, this is really interesting, because there weren’t any bird migration studies using radar in South America,’” said Ladino-Rincon. “So this is going to be a nice opportunity … to find new discoveries.”

Angelina Tang’s work on this article as a student editorial assistant was made possible by the Cornell Lab of Ornithology Science Communications Fund, with support from Jay Branegan (Cornell ’72) and Stefania Pittaluga.