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It’s a common sight up and down the Pacific Coast: a shimmering, pink-throated Anna’s Hummingbird, slurping up sweet fuel from a nectar feeder. Now, research suggests that feeding hummingbirds—a beloved backyard pastime—is also a powerful evolutionary force that has reshaped the birds themselves.  

In a published in the journal Global Change Biology last week, researchers found that an explosion in commercial hummingbird feeders in the mid-20th century drove the species to multiply across a wider range of habitats in California. And as Anna’s Hummingbirds tapped into these new food sources, they also started sporting different beak shapes better suited to gulping down human-provided nectar.

The research—which pulled data from Christmas Bird Count records, museum specimens, and historic newspaper ads—illustrates how quickly the hummingbirds adapted as people transformed their environment, the authors say. “We’re witnessing how human action is changing the organisms we see on a day-to-day basis, in almost real time,” says co-author Faye Romero, an evolutionary biologist at the University of Rochester who worked on the study as an undergraduate at the University of California, Berkeley.

Since there aren’t direct records of how feeders spread across the state, the authors took a creative approach: diving into local newspaper archives. Those records showed that around the turn of the century, early enthusiasts were testing methods for feeding hummingbirds—say, dipping a broom in sugar water and holding it up to a nest, says study author Eliza Grames, a conservation biologist at Binghamton University. But the pastime really took off after 1947 with the sale of the first patented feeder. Soon, newspaper advertisements touted feeders of various shapes and sizes—from flower-shaped tubes to big glass globes. (“Ants are eliminated; dripping is impossible,” proclaims one ad from 1957. “It is truly the PERFECT FEEDER.”)

To understand the effect of those feeders, researchers then analyzed hundreds of museum specimens dating back to 1861, propping the birds’ delicate bills up on a stand to take photos and measurements. As feeders became common, the hummers developed longer, more tapered bills that could hold a larger volume of liquid. “After World War II, you see a jump in the distribution of a bunch of these traits,” says co-author Nicolas Alexandre, a geneticist at Colossal Biosciences who worked on the study while at UC Berkeley.

Though the study didn’t analyze birds’ genes, these changes suggest that feeders are influencing the species' traits over time, Alexandre says, favoring those that help birds consume more nectar from these newly abundant sources. Instead of a shape molded to fit certain flowers, like delicate clusters of manzanita or hanging gooseberry blossoms, the post-feeder beaks seem designed to simply scoop up sugary fuel as fast as possible. “Imagine you have this unlimited, giant container of nectar that’s always available,” Alexandre says. “It makes sense to just maximize the amount of nectar you can get with every gulp.”

At the same time, beak shapes changed to improve the birds’ fighting skills. Male bills got sharper as feeders proliferated, the study showed. Hummingbirds are known to deploy their beaks as weapons to fiercely protect their food, and the high caloric value of feeders may have turned up the heat on these territorial battles, says co-author Alejandro Rico-Guevara, an evolutionary biologist at the University of Washington. “A hummingbird feeder is basically pulling hundreds of flowers together in a single spot,” Rico-Guevara says. “It’s a very tempting resource to defend.”

These nectar buffets also help explain the massive range expansion of Anna’s Hummingbirds. Once found only in a small stretch of chaparral habitat in southern California and northern Mexico, the birds today are common urban residents along the West Coast, stretching as far north as Seattle and Vancouver. Previous found that growing coastal development in the late 1800s seemed to help the birds spread, possibly because the massive planting of non-native eucalyptus trees offered food and nesting sites.

But by the mid-1900s, feeders were really driving the Anna’s Hummingbird boom across California, according to the new study, which used 80 years of Christmas Bird Count data to track the species’ spread. Out of the various ecological factors examined, feeder density—approximated by the number of newspaper feeder ads in a given county—showed the strongest relationship with the birds’ increases, says co-author Simon English, a conservation scientist at the University of British Columbia. Especially in the new northern parts of the range, “the hummingbird feeders emerged as a really strong driver of population growth,” he says. (In an interesting twist, the researchers also found that birds in colder parts of this expanded range showed their own trend in beak shape: Their bills tended to get shorter and deeper, likely to help conserve heat.)

By bringing together these different strands of data, the study adds new depth to the story of the species, showing that “not only are the Anna’s Hummingbirds moving, but they’re changing as they move,” says CJ Battey, a computational biologist at Myriad Genetics. And it offers a bit of a hopeful twist to the classic story of human development causing biodiversity loss, Battey points out.

The Anna’s Hummingbirds in California reacted to environmental changes  with impressive speed: They showed marked shifts in their beak shapes within just a couple decades, or around 10 generations, Alexandre says. Still, the researchers caution that many other species may not adapt to change as quickly or thrive in cities at all. What’s more, as people continue to modify habitats, even these limited success stories may not hold. “We’re seeing this pattern now,” Romero says, “but does that mean that organisms and animals can keep up with the pace with which we are changing the environment?”