Billions fewer birds now fly through North America’s skies than decades ago, and the rate of decline is increasing, according to a new study published in Science. The research team said the pattern is not only one of overall losses, but of losses that accelerate from year to year, with both intensive farming and warming temperatures playing major roles.
Nearly half of the 261 North American bird species analyzed showed population losses large enough to be statistically significant, and more than half of the species that were declining saw those losses speed up after 1987, the study reported. Unlike work that focuses mainly on changes in total numbers, the paper examined how quickly the declines progressed, where they were steepest, and what the declines correlated with across geography.
Marta Jarzyna, an ecologist at Ohio State University and a co-author of the study, said the losses are not only shrinking bird populations but doing so at an accelerating pace. “Not only are we losing birds, we are losing them faster and faster from year to year,” Jarzyna said. She added that, “Except for forest birds, almost every group is doing poorly,” and said the findings raise questions about how those groups could be protected.
Francois Leroy, an ecologist at Ohio State University and the study’s lead author, said the most rapid declines in the new research are in species that currently have large enough populations to avoid extinction risk in the near term. “The only consolation is that the birds that are shrinking in numbers the fastest are species — such as the European starling, American crow, grackle and house sparrow — with large enough populations that they aren’t yet at risk of going extinct,” Leroy said. He cautioned that declining abundance can foreshadow later extinction risk, saying, “the decline is somehow maybe giving a preview of what it could lead to in terms of species extinction.”
Outside experts said the findings also underscore broader ecological and environmental implications. Cornell University conservation scientist Kenneth Rosenberg, who was not part of the research, said species showing the fastest declines are often dismissed as pests. He added that if even highly adaptable generalists and human-tolerant species are losing ground, “that is a very strong indicator that the environment is also toxic to humans and all other life,” Rosenberg said.
The study’s geographic analysis pointed to where accelerating losses were most pronounced, including the Mid-Atlantic, the Midwest and California. The researchers also found differences between where declines occurred and where those declines accelerated: in general, the population declines were bigger further south, while a deeper analysis connected those declines to warmer temperatures tied to human-caused climate change.
Jarzyna said the climate and agriculture factors interact, with temperature increases and farming practices jointly shaping bird outcomes. “In regions where temperatures increase the most, we are seeing strongest declines in populations,” she said. She added that “On the other hand, the acceleration of those declines, that’s mostly driven by agricultural practices.”
Leroy said the researchers found statistical correlations between faster acceleration rates and indicators of intensive agriculture, including high fertilizer use, high pesticide use, and more cropland. He emphasized that the study could not prove that any single input caused the acceleration, but said the correlations point to agriculture as a factor. “The stronger the agriculture, the faster we will lose birds,” Leroy said. Jarzyna described the relationship as a “strong interaction” between climate change and agriculture in the way they affect bird populations, saying, “We found that agricultural intensification causes stronger accelerations of decline in regions where climate warmed the most.”
David Bird, a wildlife biologist at McGill University who was not part of the study and edited the book “Birds of Canada,” said he believed the research was done well and that its conclusions made sense. He said agriculture intensification converts more habitat to cropland and, with modern machinery, can grind up nests and eggs, while single-crop plantings reduce opportunities for birds to find food and nests. In an email, Bird said, “The biggest impact of agricultural intensity though is our war on insects,” adding that “Numerous recent studies have shown that insect populations in many places throughout the world, including the U.S., have crashed by well over 40 percent,” and he said many of the declining birds depend heavily on insects for food.
Richard Gregory, head of monitoring conservation science at University College London, said the study was “alarming” and “sobering” because of the sheer scale of the losses and because declines were accelerating. He said the paper’s results suggest people may need to change both how they live to reduce warming and how farming is practiced to reduce agricultural intensity, monoculture of crops, and broad application of chemicals. Andrew Farnsworth, an ornithologist at Cornell University who was not part of the study, also said the paper was important in part because it points to actions needed to address the drivers of bird decline.
McGill biologist Bird said birds provide benefits to people and warned that environmental damage can ripple through ecosystems. In his email, he said birds “feed us, clothe us, eat pests, pollinate our plants and crops, and warn us about impending environmental disasters,” and he said birds enrich human life. He added that, “With their songs, colors, and variety, birds enrich our lives… and recent studies show that their immediate presence actually increases our well-being and happiness and can even prolong our lives! To me, a world without birds is simply unfathomable.”