Birds haven’t been doing well in the United States with their numbers having dropped by nearly a third in just half a century. On grasslands, meanwhile, their numbers have halved.

Some scientists say they now know one of the main reasons, which has been largely overlooked thus far: the use of neonicotinoids.

Neonicotinoids are nicotine-based chemical substances that are the primary ingredients in certain insecticides and their large-scale use is partly responsible for the alarming drops in the numbers of wild birds across various habitats in the U.S., according to a team of researchers at the University of Illinois.

Specifically, an increase in the use of neonicotinoid-based insecticides can directly be linked to significant reductions in bird biodiversity between 2008 and 2014, particularly for grassland and insectivorous birds, the scientists explain in a study.

It has long been known that neonicotinoids have a harmful effect on wild bees, honeybees, butterflies and other insects, but their impacts on birds have not been studied in depth until the latest study, which is the first such piece of research undertaken on a national scale in the U.S. and took seven years to complete.

The scientists behind the study used data from hundreds of bird species in four different categories: grassland birds, non-grassland birds, insectivores, and non-insectivores.

“We found robust evidence of the negative impact of neonicotinoids, in particular on grassland birds, and to some extent on insectivore birds after controlling for the effects of changes in land use,” explains Madhu Khanna, a distinguished professor in agricultural and consumer economics at the university.

An increase of 100 kilograms in neonicotinoid usage per county, which accounted for a 12% increase on average, led to a measurable 2.2% decline in the populations of grassland birds and 1.6% in the populations of insectivorous birds. By comparison, if 100 kilograms of non-neonicotinoid pesticides was used in a comparable area, only a decrease of  0.05% was observed in grassland birds and only a decline of 0.03% in non-grassland birds, insectivorous birds, and non-insectivorous birds, the scientists say.

Worse: the harmful impacts of neonicotinoids build up over time so that 100 kilograms of neonicotinoid used in a county in 2008 reduced cumulative grassland-bird populations by 9.7% in the area within just six years by 2014.

The implication is clear: nicotine-based insecticides are a clear and present danger to wild birds.

Birds likely ingest the chemicals by feeding on crop seeds treated with them as well as by preying on contaminated insects. “Consumption of just a few seeds is enough to cause long-term damage to the birds’ reproduction and development,” the authors say.

Neonicotinoids are also more toxic to insects and persist longer in the environment, inflicting greater harm on ecosystems. The solution lies in phasing out their use in favor of less toxic alternatives, the experts note.

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