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Use of a common pesticide that impairs bird migration

by on 2019/09/30

Ever since Rachel Carson published the Silent Spring in 1962, we have known that songbirds are in great danger from pollution and human activities. Recent research has made the causes and the consequences of pesticide pollution even more evident.

A journal article published this month in Science clearly shows that the use of a common pesticide, called a ‘neonicotinoid’, is likely causing bird population declines because it is hindering bird migration. Neonicotinoids are the most widely used type of agricultural pesticide worldwide. In this study, the authors combined an experimental approach of feeding sparrows a neonicotinoid with a telemetry approach – a method of tracking individual movements in natural habitats – to follow the birds’ migratory movements. The research used the white-crowned sparrow, Zonotrichia leucophrys, which is a typical seed-eating songbird of northern latitudes. The exposure conditions that were used in the experiments were well within the dosage that a bird could realistically consume by eating even a few seeds that had been treated with a neonicotinoid.

The authors discovered that even a single exposure to low doses of a neonicotinoid caused negative effects on bird feeding and migration. Birds that were exposed to the neonicotinoid had significantly longer stop-overs, meaning that migration takes longer. The proximate mechanisms by which this effect occurred is that the birds stopped eating; they quickly lost weight from their fat reserves and this weight loss lasted for an extended period of time.  Fat reserves and fuel from feeding are critical for successful bird migration because it is very energetically challenging. The authors suggest that the longer stopover is caused by the fact that the exposed birds eat less and therefore have less fuel to continue. As every 1960s French movie star knows, nicotine suppresses appetite. Neonicotinoids are chemical sisters to nicotine and may well have the same effect on birds. Birds only continue their migration when they have enough reserves to meet the next bout of flight.

The press response to this paper has been high, especially in North America. The message and coverage has been consistent and, as far as I have seen, quite accurate and reflecting the dire state of bird health and its evidenced link to these pesticides. It seems to me that the media response is more on the side of reporting the science and biodiversity conservation now than even a few years ago, when for example in the UK media also presented the chemical industry perspective that neonicotinoids are necessary and not harmful. By now few disagree that birds are being lost at a staggering rate in the northern hemisphere; a different study in Science, on the heels of the pesticide one, found that 3 billion birds have been lost from North America in the last 50 years. Numbers in Europe are believed to be comparably drastic. Birds are indicator species for ecosystem health and evidence from this study shows that a major cause of that decline is likely a chemical that humans choose to apply.

What individual birds experience during migration has population-level consequences because it directly affects mortality and birth rates. In this case the birds were slower to continue migration, which means that they will have poorer quality breeding sites when they arrive at their destination. This is known to result in later breeding, and fewer and lower fitness offspring, with long-term effects on population viability. These consequences are evident in the results from another high profile journal article published this month, birds that are dependent on farmlands have declined in abundance by a staggering 74% over the past ca. 50 years. It is precisely these farmland birds, exemplified by the sparrows, that are most exposed to neonicotinoids and other pesticides. In fact, the authors note that some individuals may experience repeated exposure at successive feeding stopover sites, which would amplify these negative fitness consequences that were detected in their study.

The media coverage has been wide, perhaps because we are currently in a time of increasing awareness about the impact of human activities on the natural environment. The evidence that neonicotinoids inflict huge harm to bees and other pollinators has been growing. The effects are not only direct – as evidenced experimentally in this study on sparrows – but also indirect on ecosystems, for example when pesticides reduce natural insect numbers and therefore remove a valuable food source for bats, birds and other animals. Consequently, the EU, including the UK, banned neonicotinoids in 2018 (except for use in plants that live their entire life in greenhouses). The UK government says they will maintain the ban post-Brexit. However exemptions requested under ‘emergency situations’ may be a route by which neonicotinoids are still used. The fact that neonicotinoids are already banned in the EU and UK – i.e. it is now clearly accepted that they cause harm – might be the reason that the media coverage of this article has been somewhat less on this side of the Atlantic than in North America.

In North America, the timing of this study is particularly important and therefore the media attention is valuable for raising awareness on human and environmental health. Neonicotinoids are on almost all corn and canola seeds, and many soybean seeds, in Canada and are used very widely in the USA. The government agency Health Canada has proposed a phased in ban on agricultural use of neonicotinoids starting in 2019. There are few or no restrictions on their use in the USA. Trump reversed a limited Obama era ban on neonicotinoids in wildlife refuges. Hopefully the evidence that is accumulating from science will help motivate future bans across the globe.

Now it is clear that neonicotinoids are harmful to insects and vertebrates. To date, the burden of proof has clearly been laid on scientists and concerned citizens and public funds through university research to demonstrate these consequences. But other pesticides are dangerous to wildlife as well. For the sake of ecosystem health and biodiversity, let’s hope that the lessons that took so long to learn on neonicotinoids are applied more rapidly to other human actions that significantly harm nature.


Main paper discussed is:

Eng ML, Stutchbury BJM, Morrissey CA (2019) A neonicotinoid insecticide reduces fueling and delays migration in songbirds. Science, 365, 1177–1180.

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