Veterinary insecticides in wild bird nests: emerging contaminants in urban and protected forest habitats.

Veterinary ectoparasiticides widely used on companion animals often contain synthetic insecticides whose agricultural applications have been restricted in the European Union because of environmental concerns. These neurotoxic compounds can persist on animal fur and enter surrounding environments, potentially exposing non-target organisms. The Great Tit (Parus major), a common passerine in both urban and forest habitats, frequently incorporates animal fur in nest linings, thereby creating a potential exposure route for breeding adults and nestlings. We analysed fur-containing nest material collected from 63 Great Tit nests in artificial nest boxes at an urban site and a nearby protected forest in Hungary during the 2025 breeding season, sampling at mid-nestling stage and also after fledging. Using HPLC-MS/MS and GC-MS, we detected several veterinary ectoparasiticides in nest materials, including fipronil, fipronil sulfone, imidacloprid, and permethrin. Acetamiprid was found only in urban nests, indicating additional, non-veterinary environmental sources. Multiple insecticides were present in nest material, with higher contamination levels and a greater number of detected compounds in urban compared to forest nests. Residues were present at both sampling times but declined over the course of the breeding cycle. Although contamination was not associated with the measured reproductive parameters of Great Tits, our findings show that veterinary ectoparasiticides can contaminate wild bird nests, including those in protected forest ecosystems. This highlights a previously under-recognised pathway linking companion animal treatments to wildlife exposure and underscores the need to assess the ecological risks and trade-offs associated with widespread veterinary insecticide use.