Mr. Martinka, a retired state wildlife biologist and an avid bird-watcher, is part of the global ornithological network eBird. Several times a week he heads into the mountains to scan lakes, grasslands, even the local dump, and then reports his sightings to the Cornell Lab of Ornithology, a nonprofit organization based at Cornell University. “I see rare gulls at the dump quite frequently,” Mr. Martinka said, scanning a giant mound of bird-covered trash. Tens of thousands of birders are now what the lab calls “biological sensors,” turning their sightings into digital data by reporting where, when and how many of which species they see. Mr. Martinka’s sighting of a dozen herons is a tiny bit of information, but such bits, gathered in the millions, provide scientists with a very big picture: perhaps the first crowdsourced, real-time view of bird populations around the world. Birds are notoriously hard to count. While stationary sensors can measure things like carbon dioxide levels and highway traffic, it takes people to note the type and number of birds in an area. Until the advent of eBird, which began collecting daily global data in 2002, so-called one-day counts were the only method. While counts like the Audubon Christmas Bird Count and the Breeding Bird Survey bring a lot of people together on one day to make bird observations across the country, and are scientifically valuable, they are different because they don’t provide year-round data. And eBird’s daily view of bird movements has yielded a vast increase in data — and a revelation for scientists. The most informative product is what scientists call a heat map: a striking image of the bird sightings represented in various shades of orange according to their density, moving through space and time across black maps. Now, more than 300 species have a heat map of their own. “As soon as the heat maps began to come out, everybody recognized this is a game changer in how we look at animal populations and their movement,” said John W. Fitzpatrick, director of the Cornell Lab. “Really captivating imagery teaches us more effectively.” It was long believed, for example, that the United States had just one population of orchard orioles. Heat maps showed that the sightings were separated by a gap, meaning there are not one but two genetically distinct populations. Moreover, the network offers a powerful way to capture data that was lost in the old days. “People for generations have been accumulating an enormous amount of information about where birds are and have been,” Dr. Fitzpatrick said. “Then it got burned when they died.” No longer: eBird has compiled 141 million reports, or bits, and the number is increasing by 40 percent a year. In May, eBird gathered a record 5.6 million new observations from 169 countries. (Mr. Martinka’s sighting of 12 herons at once, for example, is considered one species observation, or bit.) The system also offers incentives for birders to stay involved, with apps that enable them to keep their life lists (records of the species they have seen), compare their sightings with those of friends (and rivals), and know where to look for birds they haven’t seen before. “When you get off the plane and turn your phone on,” Dr. Fitzpatrick said, “you can find out what has been seen near you over the last seven days and ask it to filter out the birds you haven’t seen yet, so with a quick look you can add to your life list.” The system is not without problems. Citizen scientists may not be as precise in reporting data as experienced researchers are, like the ones in the Breeding Bird Survey. Cornell has tried to solve that problem by hiring top birders to travel around the world to train people like Mr. Martinka in methodology. And 500 volunteer experts read the submissions for accuracy, rejecting about 2 percent. Rare-bird sightings get special scrutiny.
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