How bird feeders may be changing great tits’ beaks

Some great tits in the United Kingdom are getting long in the beak — and it may have something to do with a British fondness for bird feeders.

Parus major songbirds are thought to be relatively similar throughout Europe. But comparing DNA data from great tits in the United Kingdom with those in the Netherlands revealed key genetic differences between the two populations. The analysis, published in the Oct. 20 Science, linked those genetic differences to a slightly longer beak in U.K. birds seen over the last few decades. Since beak length is known to be associated with food availability, the researchers speculate that the U.K. great tits may be adapting to the widespread use of bird feeders in the country.

In other studies, bird beaks have been shown to be sensitive to the environment and capable of rapid change, says coauthor Mirte Bosse, an ecologist at Wageningen University & Research in the Netherlands. This new study demonstrates that even in our own backyards, small evolutionary changes may be happening, she says.

Bosse and her colleagues compared DNA data from 2,322 birds from the United Kingdom and the Netherlands. Genetic differences between the two populations suggested that natural selection, the process that drives evolution, appeared to be at work especially in DNA regions associated with the shape and structure of the birds’ beaks. In particular, the researchers found that a gene variant, COL4A5-C, associated with longer beaks in both populations was more abundant in the U.K. birds.

Measuring museum specimens revealed that U.K. birds had slightly longer beaks than those in mainland Europe — a difference of about half a millimeter — confirming the findings of a previous study. And ecological data from living U.K. birds showed that over the past 26 years, their beaks have lengthened about a tenth of a millimeter.

The researchers also discovered that the great tits carrying the gene variant for longer beaks were more successful in raising their chicks to fledglings than birds without it. That gives these birds “a reproductive advantage in the U.K., suggesting that selection on longer beaks is ongoing,” says Bosse. Why is unknown.

Previous studies have shown that changes in beak length in birds is typically associated with food availability. No significant differences in the birds’ natural diet, which includes insects, berries and seeds, exist between the two populations. But the use of bird feeders among the British is widespread. A 2009 study on domestic gardens found that nearly half of all U.K. households, about 12.6 million homes, feed the birds. It’s estimated that the United Kingdom spends about twice as much on birdseed as all of mainland Europe, the researchers say.

By monitoring great tits tagged with electronic devices, the researchers determined that birds with two copies of the long-beak gene variant tended to visit bird feeders more than other great tits with only one copy or that had genes linked to a shorter beak.

“A human influence on beak size evolution is not new; we have seen the signs in Darwin’s finches on the inhabited island of Santa Cruz in the Galápagos,” says Peter Grant of Princeton University, who studies ecology and evolution in Darwin’s finches. “But I think an association with bird feeders is new.”

More research is needed to determine whether that’s what is really driving natural selection, he says. Bosse agrees. “What it is that gives birds with longer beaks an advantage at the feeder sites?” she asks. Looking into changes in bird feeder design and the way great tits eat from the feeders could lead to answers.