science Watch / Saul Scheinbach
The Beak That Fits the Feeder
““The feeders are what really make the difference here.” – A. Rico-Guevara
Tobias/Nolan/Zuma/Rex/Shutterstock
Bird beaks are highly adaptable, readily evolving in response to changes in diet. The most famous example is the beak diversity exhibited by Darwin’s finches that allowed them to spread across the Galapagos islands and form 17 species. (See: 2004, Beak Variation in Darwin’s Finches: It’s in the Genes at https://www.hras.org/past-sciencewatch-articles.)
Now a report in the May 21, 2025 issue of Global Change Biology tells how Anna’s Hummingbird (Calypte anna) expanded its range from southern California to Canada and Alaska. Scientists led by Nicolas Alexandre, University of California, Berkely, CA. found that the species expanded northward by adapting the size and shape of its beak in response to a proliferation of feeders which began in the mid-20th century.
The team searched newspaper archives from 1861 to 2020 for mention of two factors they thought should have influenced the birds’ increase. The hummers began a slow northward expansion when Eucalyptus trees, which can flower all year, were planted in the 1930’s. In 1927 commercial feeders went on sale, but homemade ones were used even earlier. So, the scientists searched for feeder ads and mentions of Eucalyptus tree plantations and feeders. They examined Christmas count data to establish which California counties the birds inhabited over time. They sifted through human census records to see where people were moving as a proxy for feeder locations. To document beak changes they examined museum specimens collected in California.
They found that feeder use rose sharply after the first patented feeder went on sale in 1947 and there were strong positive associations of hummer abundance with feeder density and urbanization. Hummer expansion was only weakly associated with Eucalyptus density. Feeder density was the main driver for the birds’ northward expansion. The birds followed the feeders, taking advantage of a never-ending nectar source.
Analyses of 400 museum specimens going back 160 years showed that by the1950’s the birds’ beaks had already evolved in a way that boosted consumption at feeders. Beaks became longer and larger to gulp down larger volumes of liquid from a “flower” that was never depleted. Bill length was closely related to feeder density. “Imagine that you have this unlimited, giant container of nectar that is always available. It makes sense to just maximize the amount of nectar you can get with every gulp,” said Alexandre.
Why would feeder use produce longer beaks? Hummers lap up nectar with their tongues, and the scientists think that with a longer beak the tongue retracts a shorter distance to deliver the nectar to the mouth. This increases the “lapping rate” so longer beaked birds can tank up more quickly at a feeder and expend less energy. Flowers offer only a few laps, so a faster lapping rate provides little selective advantage because the bird must move on.
As feeder use proliferated males’ beaks also became pointier, enabling them to better defend their prized food supply. “Anyone who has a feeder knows that hummingbirds fight like crazy. A hummingbird feeder is essentially pulling hundreds of flowers together in a single spot. It’s a very tempting resource to defend,” said co-author Alehandro Rico-Guevara.
In addition, the team found that as the birds invaded the colder, northern latitudes their beaks became deeper and shorter, which also makes sense. Infrared imaging showed the hummers use their beaks to thermoregulate, dissipating heat in hotter climes. Shorter beaks have less surface area and lose less heat—a benefit for those living up north. “They seem to be moving where we go and changing quite rapidly to succeed in their new environment,” said Alexandre.
The scientists did not investigate whether the changes in beak morphology were due to new mutations or just selection for individuals that already had longer (or shorter) beaks. Nevertheless, their findings are a fine example of evolution in action, and one where human intervention has benefited a species rather than endangering it.