Discoveries about evolution have long been intertwined with bird beaks.
The huge variety of beak shapes among finches in the Galapagos Islands became emblematic of Charles Darwin’s theory of natural selection. The beaks had evolved over time to better suit the food available on particular islands.
In the same way, the beak of Anna’s hummingbird – Calypte anna – might become a symbol of evolution during the Anthropocene. The bill of this pint-sized bird has changed shape dramatically over the last century and a half as more people hang syrup-filled hummingbird feeders to attract the colorful, acrobatic animals, scientists reported recently in Global Change Biology.
The new discovery appears to be a “textbook” example of human-driven changes in an animal’s body, Elizabeth Steell, who studies bird evolution at Cambridge University, wrote of the new study, in which she was not involved.
Hummingbirds stand out for their unique adaptations to get food. They sport long, needle-like beaks that enable them to reach nectar buried inside flowers, and they flap their wings so quickly they can hover in mid-air to reach the blooms. The Anthropocene has not been kind to a lot of these animals. Many of North America’s hummingbirds have declined in recent years, with habitat loss and shifts in timing of when flowers bloom considered possible causes. Anna’s hummingbird is the exception. Once confined to pockets of southern California and Mexico’s Baja Peninsula, over the 20th century the bird has spread across the West Coast as far north as British Columbia.
To understand what might have driven this expansion, and how it might have been imprinted in the bodies of birds named after a 19th century Italian duchess, a group of scientists turned to an unusual combination of data archives, including old newspapers and some 400 preserved bodies of the birds dating back to the 1860s.
People have brought two potential sources of hummingbird food to California since the 1800s – flowering eucalyptus trees and hummingbird feeders. The trees, planted across the state in the 1850s, are covered in nectar-filled flowers during the winter months. Recreational hummingbird feeders, meanwhile, offered birds a year-around source of sugar water stored in bottles and accessed by tiny spouts decorated to look like flowers. Their popularity traces back to a 1928 National Geographic article that described how to “tame” the birds with such feeders. It was after World War II that they exploded as a commercial product, with the first patent in 1947.
But no one was doing a systematic census of the spread of either the trees or the feeders. To get an approximate count, the researchers combed through a digital archive of articles in California newspapers dating back to 1860. They then noted whenever either eucalyptus trees or hummingbird feeders were mentioned in advertisements or articles in a particular county’s newspapers. They compared the results to the annual Christmas Bird Count, an event designed to track bird populations, which dates back to 1938 in California. The scientists also looked at census data to track the growth in human populations around the region. Their analysis revealed that the spread of Anna’s hummingbirds tracked well with the gradual rise in popularity of the bird feeders, along with the growth in the numbers of people, suggesting the three were intertwined.
The birds’ beaks held more clues about links to bird feeders. At a museum at the University of California, Berkeley, the scientists inspected the preserved bodies of roughly 400 Anna’s hummingbirds taken between 1861 and 2020. The examination showed that over time, the beaks of the birds had become longer, an anatomical shift that would likely have enabled the birds to lap up syrup more quickly by getting their tongues closer to the source and giving the beak more overall volume, the researchers reported. The growth in beak length tracked with increasing feeder density tracked by mentions in local newspapers.
That wasn’t the only change. While these birds’ beaks overall were growing longer from one generation to the next, the researchers found the reverse was true at the northern edge of the bird’s range. There, beaks were shorter. It’s possible that’s because the colder climate pressured the birds to get better at staying warm. Birds can shed heat from their bills, as the scientists found when they used thermal imaging to see that Anna’s hummingbirds adjusted the amount of heat loss depending on the room temperature. A smaller beak would likely loose less heat, giving short-beaked birds a leg up in colder places.
While the new research casts the Anna’s hummingbird as a genius at quickly adapting to a human-shaped environment, it doesn’t explain why they are better at it than other hummingbirds, noted Steell. But these birds, with their emerald backs and rose-tinted throats, should get ready for more scientific scrutiny. They could, wrote Steell, make “an ideal model system to explore plasticity and adaptation in the face of urbanization.”
Alexandre, et. al. “Supplemental Feeding as a Driver of Population Expansion and Morphological Change in Anna’s Hummingbirds.” Global Change Biology. May 21, 2025.
Photo: Adult male Anna’s hummingbird (Calypte anna) visits a bird feeder. (CC BY-SA 3.0) via Wikimedia
