For a while now, we have known that neonicotinoid insecticides, widely used in agriculture, are implicated in the disastrous loss of honey bees in the U.S., Asia, and Europe. An important new study published in Nature last week concludes that those same chemicals are involved in the severe population decline of several species of farmland birds in Europe.
Researchers from Radboud University Nijmegen and the Sovon Centre for Field Ornithology in the Netherlands focused their study on imidacloprid, a popular neonicotinoid. Comparing the concentration of imidacloprid in surface water from 2003 to 2009 and bird counts from 2003 to 2010, they concluded that there was a correlation between higher concentrations of insecticide residues in the water and lower or decreasing populations of some species. Among those affected by imidacloprid is that talkative, balletic backyard bird, sturnus vulgaris, the European starling.
Where there was a high concentration of imidacloprid, the Dutch researchers found that bird populations tended to decline by an average of 3.5% annually. Per year, the decline may seem small, but it adds up to an ominous 20% over the 6-year span of the experiment.
In fact, for the last 30 years Europeans have been worrying about the loss of farmland birds. Theories to explain the disappearance have been plentiful. Habitat destruction has been blamed --rightly so -- but also -- with a lapse into ancient superstition -- cats. There's no doubt that domestic cats belong indoors, both for their safety and the safety of the birds, but they can't be blamed for sudden drops in avian population. After all, they didn't start hunting birds only in the recent past.
At this point, it's not known exactly how the presence of imidacloprid leads to a decrease bird population. Since the relevant species feed their young on insects, it may be that there simply isn't enough food to go around. Or the nestlings may die from eating contaminated insects. Or, since the seeds sold to farmers are already imbued with imidacloprid, granivorous species may be poisoned by the plants in the field. (What may happen, in the long term, to humans who ingest plants treated with neonicotinoid insecticides is another, as yet unknown, story.)
The researchers writing in Nature argue for a correlation between the insecticide residues and bird population decline; there isn't enough evidence yet to establish causation. This is surely a comfort to Bayer, the primary manufacturer of imidacloprid. But the reality is more complex still. As National Geographic reports, the Task Force on Systemic Pesticides (a European initiative) found that the level of neonicotinoids in the environment exceeded the lower limit at which they becomes hazardous. The chemicals are ubiquitous; they've been a marketing success; sometimes success has drawbacks.
Birdwatchers, take note that the Europeans have put some restrictions, albeit tentative ones, on the use of neonicotinoids. The U.S. hasn't. Shouldn't we do as much and more? Or should we risk losing our bees and birds?