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Sub-lethal impacts of pesticides on birds

Updated: Oct 18, 2021

When I started out in the field of contamination, I trusted that chemicals were properly evaluated for impacts and there was no need to worry about these in my day to day life. Chemists who really understood this subject would be working in companies and governments, making sure nothing happened. Over the last decade, however, I have learnt how wrong I was. Chemists, for the most part, do not understand the potential toxicological risks of the chemicals they invent and manufacture — this is not part of their training or knowledge*. The assessment of chemical impacts on ecosystems are hugely expensive and thus limited by cost, and are also usually limited to studies on very few species over short time periods, usually hours and at most days. And so, in 2020, chemicals are still brought to market with a paucity of data on their actual effect on the environment.

This absence of knowledge is having profound effects on the biosphere. Chemicals in the group of pesticides are evaluated for their direct effects on one representative species of a larger group ― where questions such as what are the effects of this chemical at this dose on this bird we have chosen to represent all insectivorous birds, are posed. The sub-lethal effects (described below) of these same chemicals seem only to be reported after a product has been on the market for a while and the impacts have been noticed at the population level of a species. It concerns me that these sub-lethal effects could be affecting bird populations, already under substantial environmental pressure, in such a way as to push them towards extinction.

Populations of most insectivorous and grassland bird species have declined substantially across the world in recent decades [1]. In the US, 74% of farmland species’ numbers have declined since the 1960s. British farmland bird populations have decreased by 83%. In Australia, bird populations over the last 30 years have been fluctuating downwards [2]. Birds feeding on insects have seen the greatest decline, with grassland species also suffering substantially, and of the insectivores, aerial insectivores around the world are seeing some of the greatest declines [1,2].

Shutterstock image by Zigmunds Dizgalvis

The use of pesticides on farms is not the only factor having an impact on bird populations in these ecosystems. Decreases are strongly correlated to a change in the way farming is practiced. No- or low-input and low-intensity farming has been replaced with high-intensity high-input farming, requiring the use of fertilisers and pesticides, whilst also reducing habitat complexity [1]. Small farms have been replaced by big ones. But the use of insecticides, and the concomitant decrease in bird populations either as a result of starvation (i.e. loss of insect prey) or secondary poisoning, has been identified as the other important factor driving bird populations downwards [3,4]. Here’s a quote from a paper by Stanton et al. (2018) which breaks down the impacts [1]:

“Of 122 unique studies investigating the effects of agriculture on farmland bird species, 51 (41.8%) reported negative effects from pesticides, 33 (27.1%) from habitat loss or fragmentation, 17 (13.9%) from mowing and harvesting operations, 11 (9.0%) from grazing disturbance, and 4 (3.3%) from reduced food availability.”

Pesticides affect birds through secondary poisoning, where they eat insects which have already consumed the pesticide neurotoxin (with all commercial pesticides a neurotoxin in some form [5]) or grain coated in the chemicals. Of the secondary poisoning effects, morbidity ― that is the effects of disease ― and mortality, being death, are the most often studied. We know from these studies that hundreds of millions (over 90 million due to carbofuran alone in one year) of birds have been killed from exposure to insecticides used to protect crops [1]. This is what we deem as a lethal effect of the chemical exposure.

Less well known are the impacts of what is called sub-lethal effects on bird populations. These alter the lives of animals in such a way as to cause declines in their populations even though they may not cause death or disease. Here are some examples of sub-lethal effects of pesticides known in the literature:

· Acephate (Organophosphate pesticide, [OPP]) ― altered orientation of songbirds (Vyas et al 1995; Eng et al 017) [1]

· Azinphos-methyl (OPP) ― increased begging of nesting swallows and decreased feeding by parents (Bishop et al 2000) [1]

· Carbofuran (OPP) ― impaired thermoregulation (Friend and Franson 1999), ataxia, dyspnea, immobility, opothitonos (Hudson et al. 1984) [1]

· Carbaryl (Carbamate) ― increased begging and decreased feeding (Bishop et al 2000)[1]

· Organochlorines ― failure to breed, nest desertion, decreased nest defence, decreased clutch size [6]

· Dichlorodiphenyltrichloroethane [(DDT) OCP**] ― thinning of eggshells in apex predatory birds

· Imidocloporid (neonicitinoid) ― reduction in food consumption, mass, and fat and probability of migration departure [7]

While some of these impacts may seem inconsequential, changes to an animals’ fat-storage, thermoregulation, and ability to navigate have real impacts on outcomes for birds. How the chemicals affect the birds would be akin to humans not having a warm home and food on the table. I consider these sub-lethal factors likely to be decreasing the resilience of species at a time when a heating climate, increased habitat destruction, and invasion by pest species, means resilience is needed more than ever. There is potential for their populations to be driven further into decline as a result.

People have responded strongly to the changes in bird populations by restricting the use of all sorts of chemicals used to protect crops. This has mainly been as a result of discovering that these chemicals have driven the death of non-target species [1]. The use of chemicals such as carbamates — including carbofuran, aldicarb, and mathomyl — and organophosphates — including parathion, malathion, and chlorpyrifos — have declined rapidly over the last decades after the death of hundreds of millions of birds. As is always the case, however, we replaced one chemical for another. Neonicitinoids are now the most popular pesticide in the world [1], and are under being scrutinised as key drivers of bird population crashes over and above the agricultural intensification that has so impacted populations before [4].


1. Stanton, R. L., Morrissey, C. A. & Clark, R. G. Analysis of trends and agricultural drivers of farmland bird declines in North America: A review. Agric. Ecosyst. Environ. 254, 244–254 (2018).

2. Birdlife Australia. The state of Australia’s birds 2015. (2015).

3. Haroune, L. et al. Liquid chromatography-tandem mass spectrometry determination for multiclass pesticides from insect samples by microwave-assisted solvent extraction followed by a salt-out effect and micro-dispersion purification. Anal. Chim. Acta 891, 160–170 (2015).

4. Hallmann, C. A., Foppen, R. P. B., Turnhout, C. A. M. Van, Kroon, H. De & Jongejans, E. Declines in insectivorous birds are associated with high neonicotinoid concentrations. Nature 9100, (2014).

5. Walker, C. H. Neurotoxic pesticides and behavioural effects upon birds. Ecotoxicology 12, 307–316 (2003).

6. Mineau, P. A review and analysis of study endpoints relevant to the assessment of ‘“long term”’ pesticide toxicity in avian and mammalian wildlife. Ecotoxicology 14, 775–799 (2005).

7. Eng, M. L., Stutchbury, B. J. M. & Morrissey, C. A. A neonicotinoid insecticide reduces fueling and delays migration in songbirds. 1180, 1177–1180 (2019).

8. Warner, J. John Warner. Personal website (2020).

* I learnt this in a webinar by Professor John Warner, who developed the field of green chemistry, which aims for the manufacture of environmentally sustainable chemicals [8]

**DDT was described as an OPP in this blog's original post. This was incorrect, as this chemical is an organochlorine, with the error correct on 7 July 2020.

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