When they hung a bird feeder in their Arlington yard, Bill and Nancy O'Brien just wanted to watch wildlife up close and do some good at the same time. But they quickly found out that interacting with nature is complicated.

First, the squirrels came to steal cracked corn from the birds. Then rats, to eat food spilled by the squirrels. A stray cat, which left behind a telltale pile of feathers, clinched it. Last summer, the O'Briens regretfully took down the feeder.

Their mixed feelings about backyard feeding are shared, for different reasons, by bird experts. The surprising fact is that the kindhearted feeding of birds is not necessarily good for them. Birds do not need feeder food, research shows, and poorly maintained feeders can expose them to disease.

The main benefit of bird feeding, advocates say, is that it provides a direct, intimate view of the natural world for more than 50 million Americans who feed the birds in their yards.

"It is not going to do significant damage. It is not going to do significant good," said Laura Kammermeier, a spokeswoman for the Cornell University Lab of Ornithology Project FeederWatch, which collects bird data from volunteers across the country. "It is something we do for ourselves, and it has a lot of educational value."

Backyard feeding is most popular in winter, when birds seem to need the most help. Some people worry that birds will suffer unless they keep the feeder filled.

But research indicates that most birds do not depend on feeders. Bird feeders supply at most a fifth of a bird's nutrition, according to a study of black-capped chickadees that researchers say also applies to other species. If a feeder is empty, birds find food somewhere else.

"I don't see in the studies that have been done that bird food is necessary for wild birds," said Stephen W. Kress, vice president for bird conservation at the National Audubon Society. "The benefits are local, and for a few species, and only as long as someone is putting it out. A lot of birds don't come to feeders, so we are deluding ourselves if we think it has a widespread effect."

Feeders can help individual birds stressed by disease or very cold weather. Black-capped chickadees with access to feeders are more likely to survive very harsh winters, according to research in Wisconsin, but they do not breed any better in the spring.

Bird feeders may have extended the wintering grounds of some birds. Researchers think that is why northern cardinals and tufted titmice, rarely seen north of Washington in winter four decades ago, now are common up to Canada. Fragile Carolina wrens are more likely now to survive harsh winters.

Some people feed birds in hopes of compensating for the loss of open land that disappears when buildings go up, which experts consider the biggest threat to birds. Wesley Hochachka, assistant director of bird population studies for the Cornell Lab of Ornithology, said that may help in winter. But in the spring, he said, most birds eat insects or worms, not feeder food, and feeding cannot make up for vanished territory they need for nesting.

Feeders generally draw mourning doves, sparrows and other common birds, not rarities. Some scientists worry that feeders subsidize starlings, pigeons and other unwanted, aggressive birds that outmuscle other species.

Duke University ornithologist John Terborgh believes feeders help purple grackles, brown-headed cowbirds and others that raid or usurp nests of native birds and the tropical birds that migrate to Washington each spring.

The strongest criticism of bird feeders is that they concentrate birds in crowds that can spread disease.

House finches that use tube feeders are more likely to get conjunctivitis, an increasingly common eye disease now spreading to goldfinches and other species. The disease was first reported among birds in Washington in the mid-1990s.

Dirty feeders also can spread fatal salmonella and avian pox. Moldy seed can transmit a fungal disease called aspergillosis.

The disease problem is such a concern that wildlife officials recently have stepped up warnings that people need to keep their feeders clean. Some feeder makers are adding similar instructions.

Feeders also enhance the risk that birds will slam into windows en route to a meal or be eaten by hawks and cats that stake out feeders as fast-food stops. A study by Project FeederWatch concluded that each feeder may kill one to 10 birds each year this way.

Some bird experts believe feeders are one reason the Cooper's hawk and sharp-shinned hawk populations are climbing. A hawk attack can be a bloodier look at nature than bird-lovers bargained for. But some accept it as part of the food chain.

"I think it's wonderful to see how some of these raptors have acclimated themselves to our suburban environment," said Paul J. Baicich, an American Birding Association editor who lives in Oxon Hill. "They are feeding at the feeder, too."

Many groups promote bird feeding primarily because they hope it will inspire people to become more concerned about protecting the environment. They want them to stop using outdoor pesticides, plant shrubs with seeds that birds eat, and support stronger environmental laws. Kress, of the Audubon Society, calls it "the gateway effect."

That is what happened to Miriam St. Clair, of McLean. At first, she only put out feeders. Now, her yard has nest boxes to house birds, a brush pile to shelter them and plants like purple coneflowers and black-eyed Susans to supply food.

"Everybody who starts feeding birds starts reading about them more," she said. "Then you start to see other things that the birds need."

For Cynthia Davis, who tends a dozen feeders and suet cages in her Annandale yard, bird feeding offers a way to teach her 8-year-old daughter about respecting and caring for wildlife. She is scrupulous about keeping her feeders clean.

Davis knows that the birds do not depend on her feeders. But she has grown to depend on their twittering presence in her yard, which is a few minutes from the noise and traffic of Seven Corners.

"You see nature at work," she said, "and I don't think you see that in a whole lot of urban neighborhoods."

A hairy woodpecker, left, and a Carolina wren share a bird feeder for a morning meal in Annandale.Cynthia Davis and her daughter, Lauren, 8, set up a feeder in their Annandale back yard early in the morning.A cardinal waits atop one of the bird feeders in the Annandale yard of Cynthia Davis while other birds get their fill.