Creeping poisoning threatens birds of prey

Especially hawks, but also red kites and sea eagles often accumulate toxins in their bodies. (Image: Oliver Krone / Leibniz-IZW)

Sad “descent”: In Germany the birds of prey are dwindling. One of the reasons for this could be creeping poisoning, according to a study. In addition to drugs and pesticides, the birds of prey mainly ingest pesticides against rodents through their prey. As the substances accumulate, critical concentrations arise in some cases, the scientists report. Scavengers and animals living near cities are particularly affected, but poisons have also been found in sea eagles.

Europe and North America are particularly hard hit – many bird species have seen significant populations. There are many reasons for this: intensive agriculture, the decline in insect populations and urbanization are causing problems for animals. Another factor is the input of various chemicals into nature, which accumulate in the birds through the food chain.

Scientists from the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin have now examined the extent to which pesticides, drugs and other poisons specifically affect birds of prey. “It is known that birds of prey are particularly sensitive to pollutants that accumulate in their bodies,” explains co-author Oliver Krone. For their study, they analyzed which substances were found in around 190 birds of prey found dead in Germany between 1996 and 2018. The bird species examined included red kites, hawks, sparrowhawks, white-tailed eagles and ospreys.

Often rodent poisons detected

As the researchers report, they found critical accumulations of rodent poisons particularly frequently: they found rodenticides in the liver tissue of more than half of the birds, and in around 30 percent they found more than one of the six poisons against rats and mice. “Our study shows that rodenticide contamination poses a threat to birds of prey in Germany,” summarize Badry and his team. Anticoagulants such as difenacoum, which are typically used against rodents in agriculture, forestry and in cities, have been found particularly frequently.

“We found the rodenticide residues in the liver tissue of more than 80 percent of the hawks and red kites examined,” says Badry. In a good 15 percent of both species, the concentrations were over 200 nanograms per gram of body weight, which, according to the researchers, is an indication of acute poisoning. “We found rodenticides in lower concentrations in almost 40 percent of our samples in white-tailed eagles, while the accumulation in sparrows and osprey was low or zero,” reports first author Alexander Badry.

Pesticide and drug residues

In addition to the rodent toxins, the scientists also detected medicinal substances in the dead birds of prey. They identified traces of the pain reliever ibuprofen in around 14 percent of the animals – these substances were particularly common in sea eagles and hawks. This suggests that the birds ingest them through fish and other aquatic prey, which in turn had previously accumulated these substances through sewage. This could also explain the evidence of antibiotics in some of the birds studied, the researchers say. They were also able to detect pesticide residues in the birds of prey: the insecticide dimethoate, which has been banned since 2019, and its metabolized intermediate omethoate were found in two red kites. In two other red kites, the scientists identified the neonicotinoid thiacloprid, which is only approved against pests and pests until 2021.

As the team points out, especially with rodenticides, the risk of poisoning increases if the birds live near urban habitats. Because the rodent poisons are not only used in stables or to control field vole and on forestry areas, but also in cities and sewer systems. As scavengers and urban birds are particularly hard hit, the sources of rodenticides along the food chain for birds of prey need to be reassessed, the scientists said. In addition, the concentrations detected in white-tailed eagles suggest that more research is needed into the sources and distribution of these substances in the environment, as the toxins appear not only on land but also in aquatic habitats, the team writes.

Source: Leibniz Institute for Zoo and Wildlife Research (IZW) in the Forschungsverbund Berlin eV, specialist article: Environmental Research, 2021, doi: 10.1016 / j.envres.2020.110602

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