11
Feb
Study Finds Increased Offspring Mortality in Pesticide-Laden Bird’s Nests
(Beyond Pesticides, February 11, 2025) In a Science of The Total Environment study, scientists test over 100 blue tit (Cyanistes caeruleus) and great tit (Parus major) birds’ nests for pesticide residues in comparison with the number of dead offspring and unhatched eggs within the nest. Fur-lined nests, from animals treated with ectoparasitic chemicals, expose birds to compounds that can impact reproductive success. The authors found fipronil, a phenyl pyrazole insecticide, in all nests, with the majority also containing the neonicotinoid insecticide imidacloprid and synthetic pyrethroid insecticide permethrin. The data shows higher insecticide levels are linked to increased offspring mortality and threaten biodiversity.
This study highlights an important exposure route that is overlooked. “Although not all bird species use fur for nest building, a substantial number do, especially cavity-nesting species,” the researchers share. “Previous research found that 74% of studied woodland bird species in central Europe incorporated fur into their nests.” Many bird species in the U.S. also line their nests with fur, such as black-capped chickadees, tufted titmice, and chipping sparrows.
While there is a wide body of science showing reproductive effects from pesticides, the researchers highlight the study’s novel design, saying, “To the best of our knowledge, no previous studies have been performed to detect and quantify veterinary ectoparasitic drugs in the fur used for nest-building and explored the potential associated effects in these bird species.” They continue, writing, “The environmental impact of insecticides used as ectoparasitic treatments for companion animals is not well understood, since they are not subject to detailed environmental risk assessment.”
Many veterinary treatments include active ingredients that are banned from agricultural use in the European Union but are still prevalent in the U.S. in both residential pest management and agriculture, increasing the risk of exposure for nontarget organisms such as birds. As the authors state: “Farm livestock and companion animals such as dogs and cats are routinely treated with chemicals such as pyrethroids (e.g., permethrin and cypermethrin) and avermectin insecticides (e.g., ivermectin). Other products, including neonicotinoid insecticides like imidacloprid (banned as a plant protection product in the EU in 2018) and the phenylpyrazole insecticide fipronil (banned for agricultural use in 2013), continue to be widely used for veterinary ectoparasitic treatments for pets.”
“These chemicals are frequently applied topically, sometimes monthly, in the form of shampoos, spot-on treatments, sprays, or impregnated collars,” they continue. “An estimated 80% of dogs and 82% of cats are treated against fleas using an insecticide at least once per year.” These include products such as Frontline®, with the main active ingredient fipronil, and Advocate®, which contains imidacloprid. (See additional resources related to pets and pesticides here, here, here, here, and here, as well as previous coverage on Seresto® pet collars here.)
Since residues from these chemicals can persist on an animal’s fur for extended periods of time, this allows toxic compounds to be dispersed into the environment and come into contact with wildlife. Certain species of birds, in particular, are exposed to these insecticides as they collect animal fur to provide an inner lining to their nests, which impacts the adults, eggs, and nestlings.
Within the study, which analyzes 103 nests collected from September to October 2020 across the United Kingdon, the authors detected 17 out of the 20 insecticides in the screening. The number of insecticides per nest ranges from two to eleven, leaving no nests pesticide-free. “Fipronil, imidacloprid, and permethrin were detected in 100%, 89.1%, and 89.1% of samples, respectively,” the researchers report.
The three active substances found in the highest concentrations include the neonicotinoid dinotefuran, synthetic pyrethroids permethrin, and cypermethrin. “Overall, a higher number of either dead offspring or unhatched eggs was found in nests containing a higher number of insecticides, higher total concentration of insecticides or a higher concentration of fipronil, imidacloprid or permethrin, suggesting that contact exposure of eggs to insecticides in nest lining may lead to mortality and lower reproductive success,” the authors conclude.
They continue, “[T]he number and total concentration of insecticides found in nests, along with the concentrations of fipronil, imidacloprid, and permethrin when considered separately, predicted a higher number of unhatched eggs and/or dead offspring for both bird species together or great tits alone.” The researchers suspect the heightened effects seen in great tits is due to the amount of hair and fur within their nests as compared to the blue tits. Blue tit nests tend to be constructed with more bark and feathers, highlighting behavior-dependent exposure variations between bird species.
The authors also note that the effects seen in this study may not represent the full impact of pesticide exposure from residues within the nests, saying: “Blue tit and great tit adults tend to clean up the nests of any unhatched egg or dead chicks before the end of the breeding season when nests where collected. Thus, we likely underestimate the actual number of unhatched eggs and dead chicks of the studied nests.”
The effects on global bird populations from pesticides has been studied since the 1960’s when Rachel Carson brought attention to the matter through her book Silent Spring. Research continues to note impacts on nontarget organisms from beneficial insects that provide ecosystems services to all wildlife that is necessary in preserving biodiversity.
This study “adds to an already long list of evidence for negative environmental effects from pesticides, in particular fipronil and imidacloprid,” the researchers say. Additional studies find:
- Systemic insecticides impact pollination, soil biota, and aquatic invertebrate communities. (See study here.)
- “Fipronil has been associated with adverse effects on birds, including reduced levels of sex hormones in Japanese quails (Coturnix japonica), and reduced immune responses in red-legged partridges (Alectoris rufa).” (See studies here and here.)
- “Exposure to imidacloprid has been linked to adverse effects in birds, causing mortality following consumption of treated seeds, with sublethal doses leading to reductions in clutch size and delayed laying date in red-legged partridges.” (See studies here and here.)
- Permethrin causes reduced body weight in chicken embryos, as well as morphological abnormalities (See studies here and here.)
In summation, the authors share: “This research reveals that ectoparasitic treatments for pets and livestock can serve as a route of insecticides exposure for birds, potentially affecting eggs and chicks in nests. This raises questions about the environmental impact of veterinary drugs, which seems to have been underestimated compared to their agricultural counterparts.”
They continue, “The study emphasizes the need for comprehensive risk assessments of active substances, considering properties like water solubility, biodegradability, environmental persistence, and toxicity to non-target species, as well as dermal toxicity.” Regulatory agencies, such as the U.S. Environmental Protection Agency (EPA), must be required to properly assess all current and future pesticides for not only impacts on beneficial species but all wildlife and human health. It is imperative that risk assessments consider the better solution to toxic chemicals that exist with organic land management.
The holistic approach that organic practices offer provides a healthy alternative to the detrimental effects of chemicals that pollute the environment and all organisms within it. Organic land management protects all species, including pollinators that are crucial to agricultural and economic productivity and food security, and also mitigates the crises of biodiversity and climate change.
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All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Tassin de Montaigu, C. et al. (2025) High prevalence of veterinary drugs in bird’s nests, Science of The Total Environment. Available at: https://www.sciencedirect.com/science/article/pii/S0048969725000737.
