Correction appended.

Cornell Lab of Ornithology conservation scientist Dr. Ken Rosenberg led an international team of 12 scientists in an analysis of decades of data on bird population — and the conclusion is disturbing. In the last 50 years, one in four birds in North America has disappeared.

Pesticide use and loss of habitat to farmland are some of the most significant contributors to the decline in bird populations, according to Rosenberg. Although scientists have known for a long time that certain bird species were threatened by human activities, this study reveals that these issues apply to birds of nearly all species.

“Seeing this net loss of three billion birds was shocking,” Rosenberg said.

The infographics show that while all bird communities in almost ecological zones have suffered, grassland birds have suffered the greatest, experiencing a 53 percent decline over the past 50 years. Some specific species have been particularly hard-hit. In the same time frame, six out of every 10 wood thrushes, three out of every four eastern meadowlarks and nine out of every 10 evening grosbeaks have vanished.

But there are two sides to this conclusion, Rosenberg said. Successful conservation efforts have meant that certain bird species, such as bald eagles, falcons and ducks, have increased in population. Falcons have increased by four times, and waterfowl have more than doubled. “These are stories of hope, resilience and success,” Rosenberg said.

One of the most important efforts of bird conservation is the Migratory Bird Treaty Act of 1918, an agreement originally between the U.S. and Canada which prohibits most killings, sales or tamperings with migratory birds. This includes a ban on the collection or sale of any part of a bird, including feathers or nests. Since 1918, the MBTA has been expanded to include Mexico, Japan and Russia, and more recent programs, such as Southern Wings, allow U.S. states to put money into international conservation projects.

Understanding where birds travel is a crucial part of effective conservation efforts, Rosenberg said. Because of technological advances in bird tracking abilities, there has been a boom in migratory connectivity — the study of migratory species through multiple life cycle stages.

Yet in recent years, the United States has moved backward in its bird conservation efforts, threatening to cancel the benefits of policies like the Migratory Bird Treaty Act, according to Rosenberg. These days, Canada is the North American leader in bird conservation.

“One of our key messages is that it’s time for the 40 to 50 million of birdwatchers in the U.S. alone to raise our voices,” Rosenberg said. This task falls heavily on the shoulders of birdwatchers because the threat is not only to game birds, who historically have been strongly defended by hunters. Now, it is common birds like sparrows and robins that are in need of conservation.

Already, Rosenberg and the study’s co-authors have been met with a “massive and overwhelmingly positive response” from individuals as well as many major news organizations. Rosenberg said one of the merits of Cornell is that the University has good mechanisms for publicity.

Birdwatchers have always been crucial to the work of ornithologists, not just in their role as activists but “the eyes of the world,” as Rosenberg described it.

Volunteers take bird counts through standardized processes, such as the Christmas Bird Count and the Breeding Bird Survey, which then goes back to scientists for analysis.

These volunteer researchers are “amateurs in the best sense,” according to Wesley Hochachka, a senior research associate at the Cornell Lab of Ornithology. “They are knowledgeable and passionate about the study of birds and bird conservation.”

“There’s something about birds that capture people’s imaginations,” Hochachka said. Because of the dedicated work of these volunteers, ornithologists have more data than other animal scientists, according to Rosenberg.

The collaboration that exists at the Cornell Lab of Ornithology has enabled the use of weather radar data to study bird population, according to Emma Greig, the leader of Project FeederWatch, which facilitates data collection among volunteer bird watchers. Weather radars pick up the biomass of migrating birds, and this entirely independent data source was used to confirm the data from bird counts — over the last 11 years, the biomass of birds migrating in the spring dropped by 14 percent.

Moving forward, important areas to study are the causes of this population decline, as well as the stage in a species’s life cycle in which these threats are strongest.

“We’re hoping this paper will raise enough awareness among people who love birds and nature,” Rosenberg said. “We need to see public outcry lead to a second wave of conservation.”

The net loss of birds was three billion, not three million, birds.