The number of birds in North America has fallen by almost one third over the past 50 years, new research reveals. According to a study published September 19th in the journal Science, the United States and Canada have lost 2.9 billion breeding adult birds since 1970, a decline of 29%. The scientists warn that this decline signals a broader ecological crisis. “It’s a strong signal that our human-altered landscapes are losing their ability to support birdlife,” said the study’s lead author, Ken Rosenberg, an applied conservation scientist at Cornell University. “And that is an indicator of a coming collapse of the overall environment.” North American birds have declined in all habitats except wetlands. However, grassland birds showed the largest magnitude of total population loss since 1970 – a decline of 53% or a total loss of 700 million adult birds in the 31 species studied. But also forest birds are dwindling with a cumulative reduction of more than 1 billion birds since 1970. Even species that occur in a wider variety of habitats, known as generalists, are part of the downward trend.

Grassland birds are the most affected because of the disappearance of meadows and prairies and the extension of farmland, as well as the growing use of pesticides that kill insects, thus depriving insect-eating birds of their food. “We see the same thing happening the world over, the intensification of agriculture and land use changes are placing pressure on these bird populations,” Rosenberg told the news agency AFP. “Now, we see fields of corn and other crops right up to the horizon, everything is sanitized and mechanized, there’s no room left for birds, fauna and nature.” The authors write that “agricultural intensification and urbanization have been similarly linked to declines in insect diversity and biomass, with cascading impacts on birds and other consumers. Given that birds are one of the best monitored animal groups, birds may also represent the tip of the iceberg, indicating similar or greater losses in other taxonomic groups.” The scientists say that steep declines in North American birds parallel patterns of avian declines emerging globally: “In particular, depletion of native grassland bird populations in North America, driven by habitat loss and more toxic pesticide use in both breeding and wintering areas, mirrors loss of farmland birds throughout Europe and elsewhere.”

For the study, the scientists from seven institutions from the U.S. and Canada combined two data sources. The first was annual surveys carried out each spring, during the breeding season, conducted by thousands of volunteers based on an identical methodology since 1970. The second source were observations from 143 radar stations which can detect the flocks of birds during migrations taking place at night. Another result of the study is that there has also been an erosion of the numbers of common birds. More than 90% of the losses come from 12 avian families, including sparrows, blackbirds, warblers and finches. “We want to keep common birds common, and we’re not even doing that,” said Peter Marra, a study co-author. “Put that into the context of the other declines that we’re seeing, from insects to amphibians, and it suggests that there’s an ecosystem collapse that should be troubling to everybody,” Marra said. “It’s telling us that our environment is not healthy. Not for birds, and probably also not for humans.” The authors highlight that their results signal an urgent need to address the ongoing threats of habitat loss, agricultural intensification and coastal disturbance, factors which will be exacerbated by climate change. These problems need to be tackled in order to avert continued biodiversity loss and potential collapse of the continental avifauna, they conclude. (ab)