https://www.scientificamerican.com/article/silent-skies-billions-of-north-american-birds-have-vanished/

More than half a century ago, conservationist Rachel Carson sounded an alarm about human impacts on the natural world with her book Silent Spring. Its title alluded to the loss of twittering birds from natural habitats because of indiscriminate pesticide use, and the treatise spawned the modern conservation movement. But new research published Thursday in Science shows bird populations have continued to plummet in the past five decades, dropping by nearly three billion across North America—an overall decline of 29 percent from 1970.

Ken Rosenberg, the study’s lead author and a senior scientist at the Cornell Lab of Ornithology and the nonprofit American Bird Conservancy, says the magnitude of the decline could significantly affect the continent’s food webs and ecosystems. “We’re talking about pest control, we’re talking about pollination [and] seed dispersal,” he says, referring to the roles birds play in ecosystems. Because it is relatively easy to monitor birds, he adds, their presence or absence in a habitat can be a useful indicator of other environmental trends. Based on the paper’s results, he says, “we can be pretty sure that other parts of the ecosystem are also in decline and degradation.”