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The world’s largest hummingbird has been flying under the radar—sort of. At first glance, the two South American birds once lumped together as the Giant Hummingbird may appear nearly identical, but genetically they’re entirely different species, according to new research that has shocked ornithologists. And as it turns out, one of these species is slightly bigger than the other, officially making it the world’s hugest hummer. The findings and genetic analysis were recently published in the .

For centuries, scientists have considered two distinct populations of Giant Hummingbird—a migratory southern population in Chile, Argentina, and Bolivia, and a non-migratory northern population in Chile, Ecuador, and Peru—to be the same species. But a mystery has long surrounded the southern birds: No one knew where they vanished to after each breeding season. In 1834, Charles Darwin himself speculated, with no evidence whatsoever, that they migrated to the Atacama Desert in northern Chile. 

In 2016, Jessie Williamson, lead author of the new study and a National Science Foundation postdoctoral research fellow, assembled an international team of researchers to solve this puzzle. “Clearly they’re migrating somewhere,” Williamson says. “We started with this idea of trying to figure out where they go and could have never predicted all of the twists and turns.”

To trace the elusive avians, Williamson engineered delicate geolocator “backpacks” weighing only 0.3 grams, ensuring they wouldn’t hinder what she believed could be a journey spanning thousands of miles. But before the team could apply the geolocators, they first had to capture the birds—a surprisingly difficult task. Thanks to their remarkable hovering skills, the Giant Hummingbirds were adept at avoiding the mist nets the researchers had stretched out across a remote Chilean valley. The process became an avian chess match, with the team constantly adjusting the nets to outmaneuver their agile opponents. “It was really funny and frustrating at the same time when they would see the net, stop, and then go over it,” says Natalia Ricote, postdoctoral researcher at the Universidad Aldolfo Ibáñez in Santiago, Chile, and co-author of the study. “They were very smart.”

On average, a single hummingbird took 146 hours of netting to capture, and by the end, the team tagged 57 birds with geolocators and satellite transmitters. As if the creatures weren’t difficult enough to catch the first time, Williamson repeated the process months later to recapture the devices and collect genomic data. But it wasn’t until her second year of attempted recapture in coastal Chile that one of the birds returned, tracker intact. “I was definitely starting to doubt whether anything would come of this,” Williamson says. “Then one evening I walked up to a net and saw a bird with the geolocator. I’m getting goosebumps now talking about it.” 

The tracked bird revealed three significant findings. First, its geolocator solved the enduring migration mystery: In the winter, the southern population of Giant Hummingbirds live among the non-migrant population in the Peruvian Andes, allowing the birds to blend in and essentially disappear. Second, this tracked bird had undoubtedly completed the longest recorded migration of any hummingbird—a 5,200-mile round trip between the Chilean coast and the Peruvian Andes. And third, the specifics of the bird’s journey through the Andes suggested deeper differences between the two hummingbird populations beyond their ranges.

During its migration, the tagged bird ascended a total of 13,000 feet, pausing at various intervals for days to acclimate its blood and lungs to the lower oxygen levels, much like human mountaineers. This behavior revealed a key evolutionary difference between the northern and southern species. The northern non-migratory Giant Hummingbird, which inhabits the Andean highlands year-round, possesses a greater total lung capacity and different blood composition compared to its southern counterpart. But it's not just the northern birds' lungs that are bigger; upon closer inspection, the non-migratory hummers measured slightly bigger in all morphological traits, including bill length, wing length, and tail length. These slight differences officially make the northern Giant Hummingbird the largest hummingbird in the world.

“It’s no surprise that people didn’t know that this was going on for 200 years, even though they’ve been aware of the birds for such an extended period,” says Jim McGuire, a professor at the Department of Integrated Biology at UC Berkeley who has extensively researched the and diversification of hummingbirds. “It’s such a subtle thing. It took a really interesting and compelling approach to figure out the story.”

Genetic analysis of the captured birds and museum specimens originally sourced from Peru and Chile revealed that the two species diverged evolutionarily millions of years ago, though it’s unclear whether the migratory trait was adopted by one species or lost by the other. But because the birds are nearly identical, specimens had been misclassified as the same species in exhibits worldwide for decades, with some dating back as far as 154 years.

“When birds are really similar in their plumage, size, and shape, they tend not to be very genetically divergent,” says senior study author Christopher Witt, professor of biology and director of the Museum of Southwestern Biology at the University of New Mexico. “It wasn’t until (Williamson) sequenced, assembled, and analyzed whole nuclear genomes, that we knew that there was no gene flow between these two species and that they had diverged a long time ago.”

About the size of a human palm,  the strikingly similar hummingbirds have relatively subdued plumage for their kind. Unlike the flashy Blue-chinned Sapphire of Peru or the Ruby-topaz Hummingbird of neighboring Colombia, both birds mirror the color palette of Peru’s Santa Eulalia Valley, where Williamson’s team, mostly local researchers who were experts in both the area and its avian inhabitants, conducted additional fieldwork. The steep Andean ridges encircling the valley where the team set up camp are camel-hued with patches of sienna and glints of green.

It was in this valley that Peruvian researcher Emil Bautista first came up with the idea for the northern species’ new Latin name: Patagona chaski, the latter portion inspired by the Quechua term for the fleet-footed messengers of the Inca Empire. “These relay runners had larger lungs and were known for their skill for transporting goods and messages throughout the Inca Empire. They were able to run super-fast at high elevations. All of these characteristics are really consistent with things we see in the northern Giant Hummingbirds,” Williamson says. “We all liked the fact that we had this awesome Quechua name, which I think honors where the birds are from. There’s a nice meshing of the landscape, history, and the characteristics of the birds.”

As for the birds’ common names, the researchers have proposed renaming the non-migratory species the Northern Giant Hummingbird and the migratory birds the Southern Giant Hummingbird. The southern birds would keep the original scientific name, Patagona gigas. While Williamson’s research and the resulting split of the species might finally put an end to one mystery, for scientists now intrigued by the evolutionary paths of these birds, new research and questions are surely on the horizon.

“It’s kind of outrageous that the largest hummingbird in the world wasn’t really recognized for what it is,” Witt says. “There are spectacular discoveries in biodiversity that are just sitting out there waiting to be found."