Setting up a bird feeder is one of the easiest ways to interact with wildlife. But could this seemingly innocent pastime be changing the very shape of our backyard birds?

It’s still too early to say for sure, says Lewis Spurgin, an evolutionary biologist at the University of East Anglia in the United Kingdom.

But he and his colleagues have discovered some truly fascinating clues that a bird called the great tit may be evolving longer beaks to access bird feeders.

"We know that evolution by natural selection produces peacocks’ tails and giraffes’ necks and that sort of thing,” says Spurgin, whose findings were published today in Science. (Read why flashier great tits produce stronger sperm.)

“But it also works in much more subtle ways that are much more difficult to observe.”

Birds of a Feather

Spurgin has always been interested in how birds have served as models to help us probe at big evolutionary questions. This is what led he and his team to start looking at DNA from two distinct populations of great tits, a yellow-and-black songbird found in the U.K. and the Netherlands.

After examining thousands of DNA sequences, the researchers found discrepancies in areas of the birds' genetic code typically associated with face shape in humans, as well as in areas associated with beak shape in Darwin finches, which have been the subject of these sorts of studies before. (See "A Darwin Finch, Crucial to Idea of Evolution, Fights for Survival.")

This made the scientists wonder if there are measurable differences between beak length in the two populations—and sure enough, there were.

“We found evidence for really rapid and recent selection for longer beaks, especially in the U.K. population,” says Spurgin.

To investigate if longer beaks were giving the birds an advantage, the team looked at ongoing studies of great tit fledgling rates. In the U.K., birds with the longer-beak gene variants fledged more chicks, on average, than those with short-beak variants.

Intriguingly, the opposite was true for the great tits in the Netherlands—likely because the British have a lot more bird feeders than the Dutch.

Why Is This Bird Feeding Goldfish?

What's more, by marking both long-billed and short-billed birds with radio-frequency ID tags and then monitoring which animals visited automated bird feeders in the U.K., the authors found that long-billed birds are more likely to take advantage of the free food than short-billed individuals.

“What we can’t say is that [bird feeders] are definitely causing the difference between these two populations,” says Spurgin. “But the correlation is intriguing, for sure.” (See "Great Tit Birds Shift Mating Schedules Due to Warming.")

Darwin’s Wildest Dreams

Arkhat Abzhanov, a researcher in evolution and developmental genetics at Imperial College and Natural History Museum in London, called the study “a particularly good example” of combining genetics with traditional research into physical attributes, such as beak shape.

However, he shares Spurgin’s caution against popping the champagne. (Here's why shy great tits flock together.)

“The missing context is, what else may be going on here besides the changes in beak shape?” says Abzhanov. “It is not clear if other traits, which could be evolving rapidly at the same time, were considered.”

For instance, it seems likely that if the birds’ beaks are elongating, then they would also be experiencing changes to their skulls, as well as the keratinous sheath that covers the beak.

In other words, the “beak does not function in isolation,” says Abzhanov, and these changes could be muddling what is really going on at the molecular level.

For Spurgin, this is all part of the fun. “I don’t imagine that Darwin in his wildest dreams could have thought that this stuff would have been happening,” he says.