Pesticide poisons songbirds

Mass extinction of gold-throats after application of the neonicotinoid imidacloprid

Dead Golden Teal - these birds died after a proper use of the neonicotinoid imidacloprid in California. © Krysta Rogers
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Underestimated Danger: Neonicotinoid pesticides could be more toxic to songbirds than previously thought, as a case from California now shows. There, the proper treatment of road trees with the pesticide imidacloprid led to an abrupt mass extinction of golden icebergs. The birds had eaten the pesticide-infested seeds that lay beneath the trees - and poisoned them, as an autopsy has confirmed.

Pesticides are considered to be one of the causes of the dramatic decline in insects, but also of field and songbirds. Under suspicion are mainly the neonicotinoids, agents that bind to receptors in the nervous system of the insects and thereby kill them. Three of these drugs, including imidacloprid, are now banned in the EU because of their harmful effects on bees in the field. For vertebrates, however, the neonicotinoids were previously considered to be less toxic.

Enigmatic bird dying

But this is obviously a mistake, as a case from California shows. On the morning of March 16, 2017, 76 street trees were treated in the city of Modesto against insect infestation. The employees of the municipal garden office mixed the Neonicotinoid Imidacloprid following the instructions on the package and sprayed it on the trunk base of the elms - as specified by the manufacturer.

On the evening of the same day, the residents of the affected streets found numerous dead golden-throated (spinus tristis) at the foot of the trees. The small songbirds weighing only about twelve grams had fallen dead on the ground, others seemed to have fallen out of the trees. To get to the root of this sudden mass extinction, Krysta Rogers of the California Wildlife Investigations Laboratory and her team collected 27 of these dead birds and subjected them to an autopsy.

Cause of death pesticide

The result: The little gold-throats had clearly not died of illness or exhaustion. "All birds were in good physical condition with adequate fat reserves and well-developed muscles, " the researchers report. Instead, the hunger apparently became the small singing birds for Verhälngnis. Almost all the golden-throats had eaten shortly before their death from the seeds of the road trees, as fragments of elm seed in their crop and stomach proved. display

Toxicological analyzes confirmed the suspicion: "A poisoning with imidacloprid was the cause of death for this golden age, " report Rogers and her colleagues. In both the goiter and the liver of the birds, they had measured elevated imidacloprid levels between 2.1 and 8.2 parts per million (ppm). For the small birds, this dose is already sufficient to cause severe symptoms of poisoning and death, explain the researchers.

Toxic effect on birds underestimated

According to the scientists, this case illustrates that the toxic effects of neonicotinoids such as imidacloprid on birds have been previously underestimated. Especially small birds very quickly take ill and deadly doses of these pesticides over seeds and soil insects, even if they have been correctly dosed and used correctly. "Mortality during the event under investigation highlights a previously overlooked risk of imidacloprid use, " say Rogers and her team.

The researchers therefore recommend minimizing the use of this pesticide, especially if trees and other treated plants carry seeds. "In addition, measures should be taken to prevent small animals from accessing the treated areas, " they say. In general, however, they generally advocate more preventive biological measures against pests instead of using pesticides.

Joint responsibility for bird loss?

In Europe, outdoor use of Imidacloprid has been banned since 2018. But the suspicion is obvious that this previously frequently used means in this country has played a role in the decline of field and singing birds. Apparently these pesticides do not only harm the birds indirectly, by making their insect food scarce - they can also poison them directly.

However, it is still unclear to what extent other neonicotinoids are similarly toxic to songbirds and other small animals. Since many of these agents have a similar mechanism of action, this seems at least not excluded. Further studies will surely have to follow. (Environmental Toxicology and Chemistry, 2019; doi: 10.1002 / etc.4473)

Source: Society of Environmental Toxicology and Chemistry

- Nadja Podbregar