Oh, dear wildlife lover, you're not helping. That bird feeder in your yard? It's a problem.

Bird feeding folk—as many as 70 percent of you and your neighbors—set out more than $3 billion worth of food for wild birds in the US every year, according to estimates from the Fish and Wildlife Service. Typically, they believe they're doing birds a favor. But, says Jack Dumbacher, curator of ornithology at the California Academy of Sciences, "they're really doing it for themselves."

He's not just being a curmudgeon. A study of the effects of urban bird feeding published today in Proceedings of the National Academy of Science finds that would-be do-gooders are primarily helping a limited number of invasive non-native species, like sparrows and pigeons, while crowding out natives. That means feeders may be harming the very species they're intended to help.

As the planet becomes increasingly paved over with buildings and roads, ecologists are turning an eye to the interplay of plants, animals, and the built environment. Josie Galbraith, an urban ecologist at the University of Auckland in New Zealand, was curious about why, around the globe, cities and towns are hotspots for non-native bird species. So she decided to take a look at where the birds were getting their food.

Spotted doves and house sparrows rule the roost. University of Auckland

Twenty-three households in Auckland volunteered for Galbraith's study: Eleven spread birdseed or bread in their gardens every day for a year and a half, and twelve homes kept the buffet at bay. A striking divergence appeared—the gardens with feed attracted fewer kinds of birds, and the populations of non-natives skyrocketed. House sparrows (Passer domesticus), native to much of Europe, the Mediterranean, and Asia, were 2.5 times more common in fed gardens than non-fed gardens, while the spotted dove (Streptopelia chinensis), a species from Asia, was 3.5 times more common.

That community disruption could have ripple effects, potentially altering how seeds are dispersed by birds, for example. The insect community could also change, and through it how local plants are pollinated, though such effects are still unknown. "But, when you consider just how many people are feeding birds around the world," Galbraith says, "it's really an unprecedented global experiment."

Backyards can, and should, be havens for birds, says Steve Kress, ornithologist and conservationist at the Audubon Society. But bird lovers need to be careful about how they construct those oases. Cities are dangerous places for animals on the wing: Windows, cats and bright lights (which can disorient migrating species) all take their toll. A way to offset some of that damage is for gardeners to create yards that are more like birds' natural homes. "If we can shift people from thinking about feeding birds to thinking about mimicking habitat," Kress says, "then we're really doing something good for birds."

Good bird habitat, says Kress, contains food and nesting material at many levels, allowing birds to live whatever lifestyle they're accustomed to. Some birds sing from the tree tops, but feed by scratching on the ground, for example. Others might feed high and sleep low. And the plants that allow for that diversity of habitats also supports the birds' diets with protein-rich insects that help chicks grow feathers. "These are powerhouse food for birds," Kress says, "and you don't get them from birdfeeders. You get them from insects that are feeding on the leaves of native plants."

Plant trees especially, Kress says. Long after you are gone, and the bulk of your good deeds have been forgotten, the oak or cherry tree you've put in the ground may still be around, feeding your feathered friends. (If you're wondering what to plant, Audubon maintains a list of trees, flowers and shrubs that are most helpful for birds and beneficial insects.)

And if you feel you absolutely must set out food or a bird bath, keep them clean, for pity's sake. (Kress suggests scrubbing them out with non-chlorine bleach a couple times a year.) Otherwise, in addition to driving away native species, your feeder can also kill birds by spreading diseases like Salmonella, trichomoniasis, aspergillosis, and avian pox. Fluffy is a deadly enough predator for the birds to contend with—don't make it any harder on them.