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Research released today (Monday 27 January) from the Â鶹´«Ã½ÉçÇøÈë¿Ú, funded by bird charity SongBird Survival, has found high levels of harmful chemicals from pet flea treatment in birds' nests lined with animal fur, is leading to an increased mortality rate of chicks.

The , shows that the fur birds use to build the inner lining of their nests contained chemicals used in pet flea treatments, such as fipronil.

The researchers collected 103 nests from blue and great tits which were lined with fur. They detected 17 out of the 20 insecticides screened. 100% of nests contained fipronil, which was banned in the EU for agricultural use in 2013, and 89% contained imidacloprid, which was banned in the EU as a plant protection product in 2018.

They found clear evidence that these insecticides are harmful to birds, eggs and chicks, with a higher number of unhatched eggs or dead chicks in nests due to a higher number of insecticides.

The researchers are now calling for the government to reassess the environmental risk of pesticides used in flea and tick treatments.

Cannelle Tassin de Montaigu, Research and Associate Fellow at the Â鶹´«Ã½ÉçÇøÈë¿Ú and lead author of the research paper says:

“No nest was free from insecticides in our study, and this significant presence of harmful chemicals could be having devastating consequences on the UK’s bird populations.

“Our research shows that based on the chemicals detected, veterinary flea and tick drugs are the most likely source of contamination. We undertook our research when it was safe to do so at the end of the breeding season, so the problem could in fact be much worse. This raises questions about the environmental impact of veterinary drugs and calls for a comprehensive environmental risk assessment of veterinary treatments.”

The chemicals are frequently applied to pets and livestock as flea-treatment, in the form of shampoos, spot-on treatments, and sprays. With the population of cats (10.9 million), dogs (10 million) and livestock (163.1 million) in the UK alone, there is a substantial risk for these treatments to enter the environment.

While not all bird species use fur for nest-building, previous research (University College Dublin and University of Aberdeen, 2020) has shown that 74% of woodland bird species in central Europe line their nests with fur.

The Â鶹´«Ã½ÉçÇøÈë¿Ú’s Professor Dave Goulson, alongside researchers at Imperial College London, conducted research released in 2024, which found that pesticides from flea treatments flow down household drains when pet owners wash their hands, polluting our rivers, and exceeding safe limits for wildlife.

This latest research further shows the damage of these insecticides on our environment and wildlife, and the importance of regulating veterinary parasitic drugs.

Sue Morgan, Chief Executive of SongBird Survival said:
“We are a nation of pet lovers and bird lovers, and it is extremely concerning to see the alarming levels of toxic pesticides in bird nests from veterinary drugs. Pet owners will be upset to hear that in trying to do the right thing to support their pets with fleas and ticks, they could be harming our ecosystem, resulting in dead newborn chicks and unhatched eggs. As pet owners, we need to have confidence that we are keeping our pets well, without devastating impacts on our wildlife.

“Our UK songbirds are in crisis. More than half of our UK songbirds are threatened or already in decline, which is why this latest research shows the importance of taking action as soon as possible. We want the government to undertake a more comprehensive environmental risk assessment of veterinary drugs.”

The British Veterinary Association recommends that vets should avoid blanket year-round flea and tick treatment policies and instead suggest individual vets to have informed discussions with their clients to risk-assess when these treatments are needed, taking into account animal, human and environmental health risks, in addition to lifestyle factors.