Bird populations are in free fall across North America. And in some hotspots, the decline is accelerating, a new study finds.
From 1987 to 2021, bird populations in California, the Midwest, and the Mid-Atlantic region declined at an accelerating rate. Across these hotspots, bird declines were associated with high-intensity agriculture, according to the study.
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Birds play important roles in ecosystems, such as dispersing plant seeds and controlling insect populations. Scientists have been concerned for decades that human activities are decimating bird populations in North America and around the world, a plight shared by many other animals. What’s special about this new study is that it reveals how North America’s decline has accelerated since the late 1980s.
“We’re not talking about decline, we’re talking about accelerated decline,” study lead author François Leroy, a postdoctoral fellow in macroecology at The Ohio State University, told Live Science. “We see this decline accelerating as human activity intensifies.”
Leroy and his colleagues mapped bird declines by studying data from the North American Breeding Bird Survey. The survey is an annual research effort by professional biologists and skilled amateurs to monitor bird populations across North America. As part of the study, participants walk along a specific route and record the birds they spot.
The researchers focused on specific routes for which they had enough data to measure the rate of decline over 35 years. These routes were primarily within the United States and included 261 bird species. Across all species surveyed, overall bird populations declined by at least 15%, with about half (122 species) recording significant declines and about a quarter (63 species) reporting accelerated declines. Common bird species such as the red-billed blackbird (Agelaius phoeniceus), house finch (Haemorhous mexicanus) and American crow (Corvus brachyrhynchos) were among the native species found to be experiencing accelerating declines.
Because the study focused on rates of decline along specific pathways, it is unclear how many birds were lost across the continent during the study period. However, previous studies have found that billions of birds have disappeared in recent decades.
A 2019 study published in the journal Science estimated that North American bird populations declined by 2.9 billion birds between 1970 and 2017. That estimate corresponds to a 29% decrease, nearly double the 15% decrease recorded in the new study. However, the 2019 study also covers earlier and longer periods where more severe losses may have occurred.
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People only started studying North America’s birds in the late 20th century, but we’ve been killing them, directly and indirectly, for much longer. For example, commercial hunting by humans drove the passenger pigeon (Ectopistes migratorius), once estimated at 3 to 5 billion individuals, to extinction in 1914.
What is the cause of “birdemic”?
New research has demonstrated that birds are suffering losses not just at the species level, but across species families and different habitats. To better understand this alarming trend, researchers compared bird data to potential factors such as temperature changes, rainfall, and land cover changes.
Accelerating bird declines have coincided with large tracts of farmland and heavy use of fertilizers and pesticides, a sign of intensive agriculture. This is consistent with European research that found that agricultural intensification has a negative impact on bird diversity.
Intensive agriculture can disrupt, alter, and destroy traditional bird habitats. The amount of land used for agriculture in the United States has not changed much since the 1980s. During this period, agriculture has become more integrated, with a decline in medium-sized farms and a shift towards large-scale farming operations, but overall land used for agriculture has slightly decreased. Therefore, the decline in birds is not only due to the size of agricultural land. However, they may be the result of changes in agricultural practices.
LeRoy said from the new research it’s not really possible to say which specific practices in agriculture are worst for bird losses. However, he noted that from previously published studies, pesticide use appears to be one of the main suspects.
A 2023 study published in the journal PNAS found that the use of pesticides and fertilizers is key to agricultural intensification and is the main pressure on declining bird populations, especially those that eat invertebrates. Most extinct bird species depend on insects for food, and insects are in rapid decline as insects are killed by the use of pesticides. Birds also directly ingest pesticides.
Mr Leroy said he wanted to know what farmers thought about the correlation between agricultural intensification and bird declines. In their study, he and his co-authors also note that agriculture can warm landscapes by reducing the amount and changing the nature of vegetation, which can amplify the effects of warming on birds.
While most of the findings were bad news for birds, there was some silver lining. For example, researchers found that forest bird populations were increasing locally, likely benefiting from reforestation of old farmland. There was also a small patch of land just north of the U.S.-Canada border where bird populations were increasing overall, and this was the only area where bird populations were increasing. But Mr Leroy said he had “no idea” why this happened.
“If you look at other parts of Canada, we’re also seeing significant declines, so that doesn’t mean Canada is doing well,” he added.
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