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It’s not every day we find ourselves with a new bird species. Recently, though, birders from San Diego to Seattle have been spotting a lemon-lime songbird with snowy white spectacles that was entirely absent from the landscape 15 years ago. The species is the Swinhoe’s White-eye, and its native range is southeast Asia. But in the past decade the birds have spread from Southern California, where they were first introduced, throughout the West Coast—and scientists think the birds could be on the brink of a population explosion that may bring them as far as the East Coast.

The Swinhoe’s White-eye may be tiny, but it belongs to a family of birds considered among the greatest speciators in the animal kingdom. Starting 2 million years ago, white-eyes quickly colonized three continents and dozens of islands, evolving from a common ancestor into 111 species, with new ones still being described. While these evolutionary go-getters never made it to North America on their own, now that they’re here, scientists are bracing for what they suspect may be a continental spread.

“The cat’s out of the bag, so to speak” says Allison Shultz, associate curator of ornithology at the Natural History Museum of Los Angeles County. Shultz is fascinated by introduced birds and was a co-author on a published last year investigating the white-eyes’ origin and expansion. Although a small population had been around since 2007, there are so many similar-looking white-eye species—a legion of lemondrops with wings—that no one was completely sure which one they were dealing with. “The first step is ‘What is it?,’ so we can better understand where it came from and what we might expect in the future,” Shultz says. The team used genetics to identify the bird as the Swinhoe’s White-eye, and they suspect it arrived to California through the illegal pet trade. 

With the species’ identity solved, Shultz and her colleagues next attempted to predict where the birds might end up. Using an approach called ecological niche modeling, the team matched climate data from the birds’ native range to areas with similar conditions in North America and discovered something striking: The white-eyes were already living in places so much cooler, drier, and more seasonal than their native range that, at least according to modeling, the places should be climatically unsuitable.  They were pushing inland to San Bernadino, across open water to the Channel Islands, and up and down the coast for more than 150 miles. In other words, after a relatively slow burn in Orange County, the white-eyes seem to be breaking out. 

Why the birds have been so successful isn’t yet known, but it’s clear that white-eyes have a knack for proliferating. They reproduce quickly—up to three times per year—and Shultz wonders whether they carry genes giving them flexible behavior or an especially adaptive immune system. The birds travel in noisy groups, are omnivorous, and have tiny brush-tipped tongues that help lap up nectar from hummingbird feeders and ornamental flowers often used in landscaping. Feeders and non-native plants seem to be a critical resource for the white-eyes, and bird baths and watered lawns could be playing a part in their success, too. Their dependence on urban plants is good news for California’s other birds; so far, scientists haven’t reported Swinhoe’s White-eyes competing for food with native species, and the birds seem to be absent in the wild outside of more urban and suburban habitats. But Shultz says that could change, because the population may be evolving rapidly.

As the species spreads, birders are tracking the expansion using the community science platforms eBird and iNaturalist. In 2018, based on eBird data, Californians reported seeing about 12,000 white-eyes across the entire state. In 2023, they reported more than 100,000. “Never before has it been possible to track where birds are in real time, in rapidly shifting and expanding populations,” says John Garrett, a project coordinator at eBird. Garrett grew up birding in Pasadena and remembers a time before the Swinhoe’s White-eye arrived. “I saw some in LA County in 2017, the first time they had been found there. I remember thinking ‘Oh boy, here they come!’” he says. “Sure enough, they have.”

Keeping tabs on introduced species like the white-eyes used to be more difficult, but in 2022, eBird revamped how it deals with non-native and escaped birds. “It was frustrating because I’d meet a lot of birders who didn’t want to record introduced birds,” says Garrett, explaining that some bird-listing rules discouraged recording exotic species because they don’t “count.” But for scientists, that meant data was slipping through the cracks. To solve the issue, Garrett’s first project was to help fix how eBird handled non-native species. Now, users can quickly distinguish between native and introduced species on their lists and on the platform’s maps (where established, non-native populations are shown in orange instead of purple), while recent escapees occupy a third category. It’s an upgrade that Garret says is keeping scientists, conservationists, and birders happy, and with the system working smoothly and more people contributing than ever, the Swinhoe’s White-eye may become the most closely tracked invasive population in history.

Given the high speed at which humans are moving birds around the world, in some ways, the white-eye’s arrival to North America feels overdue. In the late 1970s, a similar species, the Indian White-eye, escaped the San Diego Zoo, budding into a population in the low hundreds. The California Department of Food and Agriculture raised the alarm, worried about the potential for the birds to damage crops. Over several years and with a price-tag of $70,000, the team completely eradicated them. Why the state took a different course with the Swinhoe’s White-eye is unclear. (The California Department of Fish and Wildlife did not respond to an interview request in time for this story.) 

Regardless, Shultz thinks it’s too late to do anything and that the birds are here to stay. They began showing up at the museum gardens where she works just a couple of years ago, and they’re the most common species she hears in her parents’ yard in nearby Orange County. “Winter could pose a problem for them,” says Shultz about the white-eye’s ability to expand further east. “I do think we’re going to see them spreading throughout the entire West Coast very quickly.”

As the white-eyes move into new regions, Garrett notes other introduced species—parrots, pigeons, bulbuls, and munias— are thriving in cities like Houston, Miami, and Mexico City, and for the millions of people living there, they may actually be birds that people get to experience the most. “They’re part of the landscape now,” Garrett says. “If we want to understand what the future looks like, understanding them is part of that.”