Bird populations are in free fall across North America. And in some hotspots, their decline is accelerating, a new study shows.
Wild bird numbers declined at an accelerating rate in California, the Midwest and the Mid-Atlantic between 1987 and 2021. Across these hotspots, losses were associated with high-intensity agriculture, according to the study.
Birds play important roles in the ecosystem, including spreading plant seeds and keeping insect populations under control. For decades, scientists have been concerned that bird populations are falling due to human activities, both in North America and globally – a situation shared by many other animals. What is special about the new research is that it reveals how the decline in North America has accelerated since the late 1980s.
“We are not talking about the decline, but the acceleration of the decline,” study lead author Francois Leroya postdoctoral fellow in macroecology at Ohio State University, told LiveScience. “We see this decline becoming faster and faster with the intensification of human activities.”
Leroy and his colleagues charted the birds’ decline by studying data from North American Breeding Bird Surveywhich is an annual surveying effort by professional biologists and skilled amateurs to monitor bird populations across North America. As part of the survey, participants walk along specific routes and record the birds they find.
The researchers focused on specific routes with enough data to measure the decline over 35 years. These routes were primarily in the United States and included 261 bird species. Across all species surveyed, total bird abundance declined by at least 15%, with significant declines documented in about half (122) of species and accelerating declines reported in about a quarter (63) of species. Common birds – such as the red-winged blackbird (Agelaius Phoeniceus), house finch (Hemorrhoea mexicanus) and American crows (Corvus brachyrhynchos) — were among the native species found to have experienced an accelerated decline.
The study focused on the decline in specific routes, so it is unclear how many individual birds were lost across the continent during the study period. However, previous research has found that billions of birds have disappeared in recent decades.
A study from 2019 published in the journal Science estimated that the North American bird population declined by 2.9 billion individual birds between 1970 and 2017. This estimate equated to a 29% decline, which is nearly double the 15% decline documented in the new study. However, the 2019 study also covered an earlier and longer time period where there may have been more severe losses.
People first started surveying North American birds in the second half of the 20th century, but we’ve been killing them directly and indirectly for much longer than that. For example, commercial hunting by humans forced passenger pigeons (Ectopistes migratorius), a species estimated to have had one population of 3 to 5 billionto extinction in 1914.
What caused the “bird epidemic”?
The new study showed that birds suffered losses not just at the species level, but across entire families of species and across different habitats. To better understand the worrying trend, the researchers compared the bird data with potential contributing factors, such as temperature changes, precipitation and changes in land cover.
The acceleration of the birds’ decline coincided with large areas of cultivated land and high use of fertilizers and pesticides, which are signs of intensive agriculture. This follows research in Europe which has found that agricultural intensification has negatively affected bird diversity.
Intensive agriculture can destroy, change and break up traditional bird habitat. The amount of land used for farming in the United States has not changed so much since the 1980s. Agriculture has become more consolidated in that time, with a decline in medium-sized farms and a shift to larger farming operations, but there is slightly less land used for farming overall. Thus, bird losses cannot be solely due to the amount of agricultural land. However, they may be the result of changes in farming practices.
Leroy said that from the new study, it’s not really possible to say which specific agricultural practices are worst for bird loss. However, he noted that from previously published studies, pesticide use appears to be one of the main suspects.
A study from 2023 published in the journal PNAS found that the use of pesticides and fertilizers was the key to agricultural intensification as the main pressure behind most declines in bird populations, especially in birds that feed on invertebrates. Most endangered bird species dependent on insects for foodand insects are in steep decline as they are killed by the use of pesticides. Birds too consume pesticides direct.
Leroy said he would like to see what farmers think about the connection between agricultural intensification and bird loss. He and his co-authors also noted in the study that agriculture warms the landscape by reducing the amount of vegetation and changing its characteristics, which can then amplify the warming effect on birds.
While the findings were mostly bad news for birds, there were some bright spots. For example, the researchers found some local increases in forest bird populations, which likely benefited from the reforestation of old farmland. There was also a small pocket of land just north of the US-Canada border where the overall abundance of birds increased – the only region where this occurred. However, Leroy said he had “no idea” why this was the case.
“It doesn’t mean that Canada is doing better, because if you look at other regions in Canada, there were also some significant declines,” he added.
