Songbird chicks are being harmed by high levels of pesticides found in pet fur used by their parents to line nests, according to a new study.

Researchers from the University of Sussex discovered harmful chemicals from pet flea treatments in every nest they examined. They are now urging the government to reassess the environmental risks of these pesticides and consider restricting their use.

Cats and dogs are commonly treated with insecticides to prevent fleas, with many vets recommending routine preventive treatments even when pets are flea-free. However, scientists now advise that animals should only be treated if they actually have fleas.

While it was already known that chemicals from flea treatments pollute rivers and streams when pets swim, this new finding of contamination in songbird nests adds to growing concerns.

Cannelle Tassin de Montaigu, the lead author of the research paper, said: “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 study, published in Science of the Total Environment and funded by the charity SongBird Survival, found that pet fur used by birds to line their nests contained pesticides such as fipronil. The researchers analysed 103 blue and great tit nests containing fur and discovered that 100% of the nests had traces of fipronil, a chemical banned in the UK and EU for agricultural use.

Additionally, 89% of the nests contained imidacloprid, which was banned in the EU as a plant protection product in 2018 but is still widely used in pet flea treatments. The UK government is considering banning imidacloprid in agriculture but not in pet products.

The study also detected 17 out of 20 insecticides tested, revealing widespread contamination. Alarmingly, nests with higher levels of these insecticides showed more unhatched eggs or dead chicks.

This research highlights the broader environmental impact of pet flea treatments, which also enter waterways and harm aquatic life. Additionally, pet owners are at risk of chemical exposure for up to 28 days after applying the treatments.

The findings increase pressure on regulators to reconsider the use of these pesticides in pet products to protect wildlife and ecosystems. Scientists advocate for more responsible use and urge pet owners to treat fleas only when necessary.

Sue Morgan, the 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.”