Carl Schwartz, a retired journalist turned amateur data collector, searches the sky for birds during the annual Christmas Bird Count. Lindsey Konkel

Citizen Science: Part of an ongoing series about everyday people adding to our knowledge of the world around us.

It's 7:40 a.m., and the clouds are beginning to brighten from slate gray to white. The sky and the snow-covered cornfields are joined seamlessly at the horizon. The car thermometer reads 19 degrees, and there are more than two inches of snow on the ground. Kathy records the weather conditions in a yellow notebook as we slink down the road in a silver Chrysler minivan, past a small Wisconsin farmhouse. In the front seat, Marilyn leans out the passenger window with binoculars to her face.

We're here on this cold, dreary morning to count birds.

My companions and I are participating in the National Audubon Society's annual Christmas Bird Count, a continent-wide census of all the birds that spend winter in North America. The count is a serious scientific endeavor spanning three weeks, from mid-December to early January. It's the biggest, oldest wildlife survey in the world. Data collected during the count has been cited in hundreds of scientific papers.

But for the most part, the people who go out scouring the skies and bushes for birds on frigid December mornings aren't professional scientists. They're ordinary citizens without any scientific training -- just a sincere interest in nature and birds.

Without these volunteers and others like them, many large-scale environmental science studies wouldn't exist. Citizen scientists participate in projects related to climate change, conservation, and all types of wildlife and water quality monitoring. At an Ecological Society meeting in Milwaukee in 2008, more than 60 scientific papers mentioned "citizen science" in their abstracts.

All of which makes it a little easier for me to bear the cold, early-morning journey across the Wisconsin farmland. There are four of us in the van. Carl Schwartz, a retired journalist at The Milwaukee Journal Sentinel, is behind the wheel. Marilyn Bontly is on the lookout for birds, and Kathy Gallick is bookkeeping in the backseat.

They are a well-tuned, bird-spotting, data-collecting machine. For the most part, I have no clue what they are looking at each time one of them shouts "Raptor!" "Gull!" or "Woodpecker!"

Today is Carl's ninth outing of this year's count. He has traveled across the state since mid-December, tallying birds as a volunteer. Amateur birders like Carl have made the count possible since 1900, when the budding Audubon Society decided to hold an alternative to the Christmas "side-hunts" that were a popular tradition of the day. Instead of taking to the fields with guns, 27 birders from Ontario to California raised their binoculars to tally a total of 90 bird species.

The count has grown considerably since that first year. Now thousands of people across North America participate, and in the past 109 years, more than six million birds have been counted.

What happens to all of this data?

Scientists use it to answer continent-scale questions about the impacts of habitat loss and global warming on birds. Last year, a group of scientists from the National Audubon Society found that, on average, more than two-thirds of the bird species in North America are wintering farther north than they were forty years ago.

This month, the 2010 State of the Birds report, a collaboration between government agencies such as the U.S. Fish and Wildlife Service and organizations such as the National Audubon Society, provided a snapshot of how bird populations in the continental United States are faring as global temperatures rise. The report used data collected during bird surveys such as the Christmas Bird Count to determine that birds breeding in coastal and Arctic regions are among some of the most vulnerable to climate change.

"Birds are like the canary in the coal mine when it comes to climate change, because many of them fly long distances," said Daniel Niven, the lead scientist on the Audubon paper. "They respond to climate change long before we start to see a change in more sedentary wildlife and plants."

With thousands of volunteers running around each year, logging millions of bird spottings, there's the potential for utter chaos and an unwieldy data set -- but there's a method to the Christmas Bird Count's madness.

States are broken up into "count circles," each roughly 15 miles in diameter. On the day of the count, small groups of at least two to three people break up and cover every accessible road in that circle, searching for birds at feeders, on power lines, or in woodlots. Groups typically spend seven to eight hours scouring their territories before calling it a day.

Count circles and routes remain largely unchanged from year to year. The same group of birders often covers one count circle, providing consistency to the data as they become familiar with the birds typical to one area.

In our minivan, we drive with the windows down, which seems slightly insane given the temperature, but most of the birding happens from inside the car. With the windows open, the trio can see better, but more importantly, they also hear better. Some birds are more elusive than others and have to be identified by song alone.

We look for cowbirds at every cow lot and have to drive especially slow past freshly manured fields. Birds really dig the dung. We crawl brazenly up people's driveways to get looks at the sparrows at feeders and the pigeons and barn swallows on eaves.

"No one's ever pulled a gun on me for trespassing yet," Carl says.

We stop at a bend on the Rock River and get out of the car to look around more carefully. Carl and Marilyn start making "psshing" sounds. These noises are meant to simulate a bird's alarm call and often stir up curious onlookers from the bushes and trees.

"It's a whole lot easier than pulling out the screech owl," says Marilyn. In the back of the van is a Discman with little speakers that they can play a CD recording of a predatory owl. This usually gets a rise out of little birds, which immediately flutter around in panic.

"The Discman is pretty old-school compared to what a lot of birders are using these days," says Carl. Many birders now download applications to their iPhones and PDAs that can play any birdcall imaginable.

The Internet has also made it possible for people all over the world to collaborate and share their findings with scientists, and it helps scientists coordinate and compile large sets of citizen-collected data.

With the economy sagging and cuts to scientific funding, more and more researchers are turning to the public as a source of labor. But citizen science isn't always cheap, says Jonathan Silvertown, an ecologist at the Open University in the United Kingdom. Silvertown uses citizen-collected data in his ongoing project, Evolution Megalab, which allows the European public to participate in a hands-on study of snail evolution.

It takes a lot of resources -- hours and money -- to design a project around a large data set and then coordinate hundreds or thousands of volunteers and organize and validate all their data.

Then there's the question of whether citizen-collected data is as good as scientist-collected data. Niven, the Audubon scientist, thinks it can be. He acknowledges that the data isn't always perfect, but robust statistical analyses can account for most human error.

"A lot of times these amateurs know their birds better than professional ornithologists," Niven says. But volunteers don't have to be experts, he emphasizes. Christmas count coordinators pair newbies (such as myself) with people like Carl, Kathy, and Marilyn, who have more than 75 years of birding experience between them.

Carl pulls the van over to the side of the road so we can investigate a woodlot dominated by poplar and ash trees. Across the street is a junkyard cluttered with rusty farm implements and old pick-ups.

Carl first remembers watching birds as a young boy at his mother's feeder. But he says he didn't become "a real birder" until a vacation to Florida in the mid 1980s. The herons and egrets he saw wintering there captivated him. When he realized that many of the same birds made their homes in Wisconsin during the summer, he signed up for some workshops and field trips hosted by local groups to learn the birding basics. He hasn't stopped since.

After 38 years as an editor at the newspaper, Schwartz called it quits last July. In his retirement, he plans to spend more time birding and collecting data for scientific studies. He now participates in bird censuses across the state, making valuable contributions to the way both scientists and the public understand the natural world.

"I know that this data will be used for research purposes," he says. "For me, it fulfills this sense that I am contributing to the greater good."