Potentially Toxic Elements in the Avian Nervous System: From Bioaccumulation to Neurotoxicity and Ecological Consequences

Environmental contamination by potentially toxic elements (PTEs) poses a persistent threat to wildlife, yet their accumulation and neurotoxic effects in the avian nervous system remain poorly understood. Although birds are widely recognised as valuable bioindicators of environmental pollution, most biomonitoring studies have focused on conventional tissues such as the liver, kidney, blood and feathers rather than on neural tissues, despite the high susceptibility of the nervous system to toxic injury. This review synthesises current knowledge on the bioaccumulation and neurotoxicity of PTEs in the avian nervous system, with emphasis on the brain, retina, optic pathways and spinal cord. Mercury remains the best-characterised avian neurotoxicant, whereas increasing evidence indicates that lead, cadmium and interactions with essential elements also contribute to neural dysfunction through distinct but partially overlapping mechanisms. Taken together, the available evidence indicates that neurotoxicity cannot be interpreted solely based on total elemental concentrations but also depends on chemical speciation, regional and cellular localisation, disruption of neural metal homeostasis and the intrinsic vulnerability of different neural tissues. Despite increasing evidence of PTE accumulation in neural tissues, important knowledge gaps remain regarding tissue-specific susceptibility and the links between neural damage, behavioural impairment and ecological performance. This review highlights the avian nervous system as an underexplored but biologically relevant target of environmental PTE exposure and highlights the need for more integrative approaches combining chemical, histopathological and functional assessments to improve avian biomonitoring and ecological risk assessment.