NOAH Statement Regarding The “High prevalence of veterinary drugs in bird’s nests” Research Paper

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NOAH Statement Regarding The “High prevalence of veterinary drugs in bird’s nests” Research Paper

The animal health industry is committed to a healthy environment for all, guided by the principles of ‘One Health’ that links human, animal, and environmental health. NOAH members believe antiparasitic veterinary medicines remain essential veterinary medicinal products that play a key role in preventing animal health and welfare problems from arising, while also protecting human health from zoonotic disease.

A recently published scientific article, titled “High prevalence of veterinary drugs in bird’s nests”, raises concerns about the impact of compounds commonly used in veterinary antiparasitics on the health and behaviour of songbirds and their offspring.

While NOAH supports ongoing research about the use of antiparasitic medicines, any such research must be based on sound science. NOAH is aware of several limitations and methodological issues associated with this paper which warrant a critical examination to avoid overstating the impact of their findings and causing unnecessary concern among pet owners and veterinary professionals.

Summary:

• The paper does not show a link between the use of pet antiparasitic medicines and the death of songbird chicks.
• Furthermore, the authors state clearly that “no results were significant for blue tits” and “no statistical impact” on dead chicks in great tit nests.
• We do not argue that traces of spot-on medicines cannot be found in pet fur, and therefore it is possible that such fur can be found in bird nests. However, the paper states there is no causality proven and there are many causes of baby bird mortality and unhatched eggs.
• Unhatched eggs and dead chicks are a natural part of a songbird’s lifecycle, especially later in the season when the health of the females is much reduced (Nestwatch, 2025).
• Both blue tit and great tit numbers have been increasing since the 1950s and have been stable since 2017 (British Trust for Ornithology, 2025a; 2025b).
• NOAH is disappointed by the reporting of the paper in the Guardian and other media which makes sensational headlines that could make vets, Suitably Qualified Persons and pet owners believe they are impacting the survival of songbirds; the findings of this study do not support this suggestion.
• Antiparasitic veterinary medicines allow us to care for our pets and our health. NOAH and our members support the responsible use of these products for the good of our pets, pet owners and the environment. 

Sample Size

• Out of the 237 nests collected, only 103 had the required 25 mg of fur for analysis. The majority of nests (57%) lacked sufficient hair and were not tested, suggesting that most nests likely did not have detectable fipronil residue, contrary to the article’s implication that all nests (100%) contained fipronil. Additionally, researchers collected 168 blue tit and 69 great tit nests, but the graphs show approximately 34 and 39 entries respectively. This discrepancy is not due to duplicates; there is no indication within the paper how this data has been handled.

Methodology

• The article provides an image of a dog being treated, with its fur hung in a garden for birds to use as nesting material. Since this isn’t detailed in their methods, it is unknown if some of the nests contained dog hair collected in this way.
• The paper doesn’t clarify if the field experiment was conducted in a controlled environment with consistent protocols, raising the possibility of inconsistent observations and handling of nests.
• In the methodology, nests were sent by volunteers who received a questionnaire via Twitter or through the British Trust for Ornithology (BTO), asking for details like the number of unhatched eggs, number of chicks, and number of dead chicks. Volunteers were instructed to leave blank any questions they couldn’t answer, and the authors noted that missing information was excluded from the analyses. Given the volunteer collection method, it’s unlikely that nests with dead chicks or unhatched eggs were not reported. A more plausible explanation is that empty questionnaire boxes indicated no unhatched eggs or dead chicks, potentially excluding 80% of blue tit data and 43% of great tit data, skewing the study towards nests with these observations.

Presence and Concentration of Compounds in Nests

• The detected levels quoted in much of the coverage are the higher levels that were detected in some of the samples; in the majority of cases the levels found in the nests were actually very low, in many instances they were many folds less than the median concentration. This approach inflates perceived risk by emphasising high incidence with sensitive detection methods and likely outlier samples (e.g. hairs from a recently treated animal).

Composition of Fur in Nests

• The research paper lacks information on the mass fraction of fur relative to the whole nest mass and does not clarify if the fur is in direct contact with the eggs, chicks, or birds. Fur typically represents a very small portion of the nest mass, so the concentrations of the detected parasiticide in the entire nest are likely much smaller than the presented data.

Unhatched Eggs and Dead Chicks

• The study assessed the presence of insecticides in the fur used for nest building but did not evaluate pesticide concentrations in unhatched eggs or the dead chicks. No direct causality of death nor impact on nesting health can be inferred from the data, consistent with the author’s own conclusions: “It is not possible to assess whether we would expect the concentrations of insecticides found in nests in our study to harm eggs or chicks”.
• A wide range of deaths (0-8) was observed in nests with little to no pesticide residues. Moreover, inconsistencies in nest handling and environmental factors may account for the number of unhatched eggs and dead offspring, making direct correlation with pesticide concentrations in nest materials unverifiable. The results do not support the hypothesis.
• Dead chicks were observed at all imidacloprid concentrations, particularly at low levels, making statistical analysis problematic and resulting in a weak correlation with no significant impact on nest viability.
• Finding dead chicks or unhatched eggs is common, especially since nests were collected in late Autumn when female birds may be physically depleted from previous egg-laying. Additionally, songbird eggs can fail to hatch for various reasons, including lack of fertilisation, extreme weather, eggshell damage, improper incubation, malnutrition, stress, predation, and the age or inexperience of the breeding pair. The British Trust for Ornithology estimates that only 20% of songbirds that hatch will reach maturity (Nestwatch, 2025c).

Lack of Significant Findings in Blue Tits

• None of the results were significant for blue tits, which comprised the majority of the nests studied. Despite similar pesticide concentrations in the hair of both species’ nests, the authors continue to speculate on the impact of veterinary pesticides on the birds.

Great Tit and Blue Tit Population Ecology

• The authors begin by stating that pesticides have driven bird population declines since the 1960s, but the cited references suggest this may be due to reduced invertebrate food sources on treated crops. Notably, great tit and blue tit populations have increased over the past 50 years and have remained stable since 2017 (Department for Environment, Food & Rural Affairs, 2025).
• The authors mention an average hatchability of 72% for several bird species (ranging between 25-100%), but do not specify where blue tits and great tits fall within this range. The authors acknowledge that offspring mortality and hatchability may be influenced by factors other than insecticides in nests.
• If the range for the number of dead offspring and number of unhatched chicks for low pesticide residue is considered normal variability, the outlier nests with the higher concentrations fall within the normal background range for these endpoints. The results do not support the hypothesis.

Other Unknowns and General Comments

• The study did not provide data on the number of eggs laid in each nest, or whether contact/airborne exposure would cause toxicity in the birds, or if ingestion is necessary for an effect. It also did not address if the adult birds or offspring ingested any of the hairs, the amount of hair in each nest, or how hair correlates to pesticide concentrations.
• The authors made several overarching assumptions. They incorrectly state that fipronil concentration in nests fall within the range found on dogs 4 weeks after fipronil treatment, which is not true based on their own data.
• Although the authors claim to have shown a correlation between antiparasitics and songbird viability, they acknowledged that this does “not necessarily reflect causality; it is plausible that the insecticide concentration and offspring mortality could be responding to some other unknown factor”.
• The authors cite published evidence that “exposure treatments in nests could have short-term positive effects by reducing nest parasitic load, which in turn could benefit breeding success”.

Conclusion

To summarise, NOAH and the wider animal health industry recommends against using this recent research and the associated media coverage to inform policy on use of companion animal antiparasitic medicines, due to its limitations and other methodological issues as outlined above.

All veterinary medicines, including flea treatments and other antiparasitic veterinary medicines, marketed in the UK are subject to strict regulations. They must undergo an independent scientific assessment by the government’s Veterinary Medicines Directorate (VMD), before being allowed on the market and to be used by pet owners. This independent regulatory assessment  and oversight considers the product’s quality, efficacy, and safety, including safety for pets, people, and the environment. The benefit risk balance of products remains under ongoing, continuous regulatory review to ensure that it is appropriate for these products to remain on the market, for the health and welfare of animals.

NOAH members encourage vets, SQPs and retailers to ensure that pet owners are aware of the instructions that are included on product packaging. Antiparasitic veterinary medicines are essential products for animal health and welfare. Correct use of these products can ensure that these products can be used safely and effectively with any risks being mitigated against.

References

British Trust for Ornithology (2025a). Blue Tit. Available at: Blue Tit | BTO – British Trust for Ornithology (Accessed 17/03/2025)

British Trust for Ornithology (2025b). Great Tit. Available at: Great Tit | BTO – British Trust for Ornithology (Accessed 17/03/2025)

British Trust for Ornithology (2025c). Biology. Available at: https://www.bto.org/understanding-birds/birdfacts/about-birdfacts/biology

Department for Environment, Food, & Rural Affairs (2025). Wild bird populations in the UK and England, 1970 to 2023. Available at: Wild bird populations in the UK and England, 1970 to 2023 – GOV.UK (Accessed 17/03/2025)

Nestwatch (2025). What should I do with unhatched eggs or dead young in the nest?. Available at: What should I do with unhatched eggs or dead young in the nest? – NestWatch (Accessed 13/03/2025)