Eating to survive...or not? feeding compensation and toxicity in a songbird exposed to thiamethoxam trated seeds

Abstract

Thiamethoxam (TMX) is a neonicotinoid insecticide widely used for seed treatment in agriculture. Birds can be exposed to TMX by ingesting unburied seeds after sowing. This study evaluated the toxicological effects of such an exposure scenario in a common farmland passerine Agelaioides badius. Birds were subjected to a 21-day dietary exposure of TMX-treated seeds at 0 (control), 0.027 (low), 0.33 (medium), and 4.3 (high) g TMX/kg seed (N=8 per group), representative of typical agricultural practices. A dose-dependent increase in seed consumption was observed in the latter stages of the experiment, suggesting compensatory feeding that amplifies toxicant intake. Notably, the high-dose group exhibited 50% mortality, an unexpected finding of concern given TMXs classification as of low acute toxicity. High-dose birds displayed significant body mass reduction (-20.5%) and impaired anti-predator responsiveness. Furthermore, general activity patterns were altered across all treated groups. At a physiological level, TMX exposure in the high-dose group induced reduced hematocrit and decreased polychromasia. Tissue-specific modulations in enzymatic biomarkers, including elevated cholinesterase and glutathione S-transferase activities, indicated oxidative stress and cholinergic alterations. These findings demonstrate that environmentally-relevant ingestion of TMX-coated seeds can elicit sublethal and lethal effects in granivorous birds, potentially posing a significant risk to farmland bird populations. Thiamethoxam effects on birds are little studied due to its reported low acute toxicity. This study demonstrates that ingestion of thiamethoxam-treated seeds causes multilevel biological effects and compensatory feeding, underscoring ecological risk for farmland bird populations.