Every year, Americans dish out more than $4 billion in food aid to birds. Does it help, or hurt?

I have an ambivalent relationship to my birdfeeder. For weeks at a time I’m a diligent provider, doling out the seed, beckoning every nuthatch and sapsucker within a two-mile radius. Then I slack off. The feeder swings empty, and I rationalize my neglect with a Darwinian argument: By feeding the birds, I only weaken their ability to survive in the wild, without me. I am lazy so they won’t be.

As it turns out, ecologists share my conflict. Feeding the birds is a popular habit: 43 percent of Americans (and 75 percent of Brits) do it, backed by a $4.5 billion dollar industry. That’s a lot of bird welfare. But to what end? Many studies have tackled the question, but comparing them — different species, different locations — is a challenge. Recently Gillian Robb and Stuart Bearhop, biologists at the University of Exeter, and their coauthors reviewed the literature; their conclusions are a mixed bag. 

First the good news. Most studies have found that birds that visit feeders (especially in winter) eventually benefit. The adults breed earlier, which increases their odds of laying another clutch of eggs later in the season. When woodland birds have access to supplemental food, their chicks fledge in greater number, perhaps because feeder-fed moms can spend less time foraging and more time incubating and protecting eggs. Robb and her colleagues conducted a field experiment in which they set out birdfeeders for a species of titmouse. Their results were typical: by the second year, the fed birds were surviving longer, breeding earlier, and producing more young.

But for some birds the backyard buffet can become an ecological trap. Florida scrub jays that live near suburban feeders breed earlier — but doing so causes them to fall out of sync with the life cycles of the bugs they feed their chicks; the bird’s overall breeding success actually drops. In Finland, some titmouse populations are so dependent on winter feeders that their numbers can’t be sustained by natural sources alone.

Bird feeders also affect bird behavior, in unpredictable ways. One species (black-capped chickadees) becomes much less territorial, while another (Carolina wrens) become more so. A third (varied tits) is less likely to join mixed flocks — a habit thought to aid the search for food during lean times. Congregating at feeders may encourage the spread of avian diseases like mycoplasmal conjunctivitis, which has hit the American house finch in recent years. One recent study found that Europe’s blackcap warbler may be diverging into two separate species: one that spends the winter in Britain, thriving by the grace of birdfeeders, and a second that migrates south to Spain as it always has.

Our tons of food aid are surely reshaping food webs and bird ecology on a wide scale; what’s unclear is how. “You have to think that all this feeding has huge potential effects,” Bearhop says. One big unknown is how birdfeeders affect the many birds that don’t visit them. Jays and crows spend most of their time around human food sources (which may partly explain the rise in these birds’ numbers). But these birds — crows especially — also prey on other birds’ eggs, possibly depressing those populations. Bearhop wonders whether the unnaturally high numbers of birds that are drawn to, and breed near, winter feeders may deter spring migrants from returning. These areas then become dominated by fewer species, largely dependent on us. My welfare helps the birds that I see — but at the expense of which others?

Absent strong evidence of any negative impacts, Bearhop says he will continue to dutifully restock his own birdfeeder, and thinks you should too. His own interest in zoology was sparked in part by seeing birds close up at his boyhood feeder. “My feeling is, feeders are probably the way that most of the public engages with wildlife. This is one way that we gain that link back to where we all came from. In that sense, they perform a really important role.” 

Personally, I’m sticking to my unreliable regimen. My birdfeeder is a paradox in a nutshell: I want and need nature, but I do not want it to need me; if it did it would feel unnatural. Given those terms, the best thing I can do for my birds is keep them on their toes.

Alan Burdick writes for numerous publications including The New York Times Magazine, Harper’s, GQ, and Natural History. His book Out of Eden: An Odyssey of Ecological Invasion was a 2005 National Book Awards finalist.