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Conservation Science for a Healthy Planet

Urban Bird Feeders Are Changing the Course of Evolution

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Emily Voigt

….According to experts, feeding birds is probably the most common way in which people interact with wild animals today. More than 50 million Americans engage in the practice, collectively undertaking an unwitting experiment on a vast scale. Is what we’re doing good or bad for birds? Recently, researchers at the Cornell Lab of Ornithology sought to answer this question, analyzing nearly three decades’ worth of data from a winter-long survey called Project FeederWatch. Preliminary results suggest the species visiting our feeders the most are faring exceptionally well in an age when one-third of the continent’s birds need urgent conservation. Still, what are the consequences of skewing the odds in favor of the small subset of species inclined to eat at feeders? What about when the bird we’re aiding is invasive, like our house finch?…

…..Traditional wisdom dictates that birds don’t become dependent on a free lunch. The idea traces to the mid-1980s when the wildlife ecologist Stanley Temple and his then-student Margaret Brittingham color-banded several hundred black-capped chickadees in the Wisconsin woods, mounted two specially designed feeders, then laboriously counted the sunflower seeds each bird ate. To the scientists’ astonishment, the chickadees obtained only 21 percent of their daily energy requirements from the feeders. The findings suggest that birds haven’t evolved to rely on a single ephemeral food source; rather, they’re constantly sampling the environment, forever scanning for backup options in a changeable world.

When Temple and Brittingham removed a feeder that had been stocked for more than 25 years from one of their study sites—a nature center at Devil’s Lake State Park—the chickadees there fared as well through the winter as those in the remote reaches of the reserve, where birds had never even seen a feeder. In other words, birds don’t forget how to forage just because they’re receiving handouts. Only in especially severe weather did feeders appear to undoubtedly help: chickadees with access to one had nearly double the chance of surviving the harsh Wisconsin winter—the difference coming down to a few frigid days….

….If the classic studies on rural chickadees found they didn’t become dependent on feeders, how much did that tell us about birds eking out an existence on the mean streets of Greenwich Village? The question is more relevant today than ever before, in an age when half the human population now resides in cities for the first time in history, a proportion that’s rapidly on the rise. Over the coming decades, the future of many wild animals will depend on their ability to adapt to an increasingly urbanized world. At the library, I searched out a book called Avian Urban Ecology, which said that little is known about the impact of feeding most birds in cities. There is, however, one stunning exception….

….If Darwin saw his theory of natural selection illuminated in the finches of the Galapagos, Badyaev is looking deep into the house finch to resolve a question he feels Darwin never answered: How does life maintain evolvability, the facility to change and adapt to the future?

….In the Southwest, for example, where Badyaev is a professor at the University of Arizona, house finches live on their native turf and nest on the cholla cactus, females shielding eggs against temperatures hot enough to hard-boil them. Meanwhile, some thousand miles to the north, where Badyaev earned his doctorate at the University of Montana, the same species hatches its young under snow cover, having undergone a transformation involving everything from hormonal signaling to eggshell architecture. To understand such radical metamorphoses, executed over a few short decades, his team has been measuring nearly every conceivable metric in hundreds of thousands of banded finches across dozens of populations, yielding one of the largest studies of wild birds in the world.

….Comparing the beak sizes, bite forces, and diets of the two populations, the researchers showed that the urban finches rely so heavily on feeders that their beaks have adapted: they’ve become longer and deeper to accommodate the sunflower seeds typically on offer, which are much larger and harder than the small cactus and grass seeds that rural finches eat. This adaptation has altered not only how urban males sing, but also what urban females prefer in a mate. It’s a pattern that Badyaev has since found in other places where finches live in the shadow of humans, the same large beaks arising from a surprisingly diverse array of developmental pathways. Such varied routes to an identical end—a beak strong enough to crack sunflower seeds—may be one way that nature hides variability from the swinging axe of natural selection….

….so should we feed birds? By doing so, we’re almost certainly changing them. A recent study in Science found that the beaks of great tits have evolved to be longer in the United Kingdom than in continental Europe, possibly due to the popularity of feeders in England. What’s more, species from hummingbirds to northern cardinals are wintering farther north than they have in the past, though whether as a consequence of feeders, the warming climate, or both remains a largely open question. (To say nothing of the garbage and waste grain from agriculture that we feed many birds without intending to.) Precisely because we’re altering nature so radically and in so many ways, bird feeders are bound to seem comparatively innocuous. Meanwhile, proponents argue that they connect us to wildlife, opening our eyes to the world beyond our backyards, including those species most in need of our help, such as seabirds and tropical birds….

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